If a letter or a lawsuit says your product infringes a patent, the first feeling is usually alarm. That is normal. What helps most is a short list of things to do right away, a clear way to read the papers, and an honest picture of your options. This guide gives you all three. It is general information, not advice about your situation, and every case turns on its own facts. Not every letter is about a patent. Some assert copyright, trademark, or trade dress rights, and some assert several at once. The first steps here apply to all of them: find the deadline, preserve your records, check your contracts, and read the sender's public record. The law that decides each kind of claim is different, so confirm which right the letter actually asserts before relying on the patent-specific parts below.
Find the deadline first
Do not ignore it. A demand letter and a complaint are different things. A letter is a private demand: it does not start a case or create a court deadline, even if it names a date for you to answer. A letter also does not mean no lawsuit exists. A patent owner can file a complaint first and serve it later, so search the federal dockets for your company's name with the PACER Case Locator, described below, to see whether a case is already pending. A complaint filed in federal court starts a lawsuit, and once it is served on you, the clock is real. Write down the date you were served, and keep the envelope or the delivery record.
Under Federal Rule of Civil Procedure 12(a)(1)(A), a defendant must serve an answer within 21 days after being served with the summons and complaint; a motion under Rule 12 served within that time changes the answer date (Rule 12(a)(4)). If the plaintiff instead sent you a request to waive formal service under Rule 4(d) and you return the waiver on time, the answer is due 60 days after the request was sent (90 days if it was sent to you outside any judicial district of the United States). Waiving service does not give up any objection to personal jurisdiction or venue (Rule 4(d)(5)). If you and the plaintiff are both located in the United States and you refuse to waive without good cause, the court must make you pay the expenses of formal service, plus the fees of any motion needed to collect them (Rule 4(d)(2)). The court may extend a deadline for good cause, and asking before it runs out matters (Rule 6(b)(1)). Do not treat an extension as granted until it is. If a defendant fails to plead or otherwise defend and that failure is shown, the clerk must enter a default (Rule 55(a)), and the plaintiff can then seek a default judgment (Rule 55(b)).
If the papers come from the U.S. International Trade Commission, you are a respondent in a Section 337 investigation, a separate proceeding in which a violation can lead to an order excluding the products from entry into the United States (19 U.S.C. 1337(d)). Under the Commission's rules, respondents generally have 20 days from service of the complaint and notice of investigation to file a written response, unless the notice or the administrative law judge orders otherwise; when the complainant has also moved for temporary relief, the response can be due within 10 days (19 C.F.R. 210.13(a)). Treat that date as urgent.
A letter is not nothing, either. If a patent owner, or someone selling under it, sells a product covered by the patent and has not marked it with the patent number, the Patent Act generally allows damages only for infringement after the accused infringer was notified and kept going (35 U.S.C. 287(a)). A letter can be that notice. A court can also increase damages up to three times the amount found (35 U.S.C. 284), although the Supreme Court has said that enhanced damages are "generally reserved for egregious cases of culpable behavior" (Halo Electronics, Inc. v. Pulse Electronics, Inc., 579 U.S. 93 (2016)). On the other side of the ledger, the statute says that not getting, or not presenting, a lawyer's opinion about the patent may not be used to prove willful infringement or intent to induce infringement (35 U.S.C. 298).
Preserve, pause, and check your contracts
Keep everything. That means the letter or complaint, your emails about the product, design files, source code, sales records, and older versions of the product and its manuals. Turn off any automatic deletion that could reach those records. The federal rules allow a court to act when electronically stored information that should have been preserved in anticipation of litigation is lost because a party failed to take reasonable steps to keep it (Rule 37(e)).
Pause before you call the sender. It is natural to want to explain that your product is different, or that you are a small business. But what you say about your product, your sales, or when you first heard of the patent can come up later. Federal Rule of Evidence 408 bars using settlement offers, and statements made in compromise negotiations, to prove or disprove the validity or amount of a disputed claim. It is a rule about what a court admits as evidence, not a promise that the conversation stays confidential: the court may admit the same evidence for another purpose (Rule 408(b)), and not every conversation is a compromise negotiation. If you need to reply before you have advice, a short note saying you received the letter and are looking into it, without discussing the product, the patent, or your sales, confirms receipt and little else.
Check your contracts. If the accused product, or a part of it, came from a manufacturer, distributor, or software vendor, your purchase orders, supply agreements, or platform terms may include an indemnity, a promise by the seller to defend you or cover your losses from an infringement claim. Find any such clause now and read it for notice requirements and deadlines. Even without a written clause, there may be a warranty. Section 2-312(3) of the Uniform Commercial Code, a model law that states enact in their own commercial codes, often with changes, provides that unless the parties agreed otherwise, a seller that is a merchant regularly dealing in goods of the kind warrants that the goods are delivered free of any rightful infringement claim, while a buyer who furnished the specifications must hold the seller harmless. That warranty covers sales of goods, it is not a promise to defend you, and under the model code a buyer who is sued must notify the seller within a reasonable time after learning of the suit or lose any remedy over against the seller (U.C.C. 2-607(3)(b)). Check your own state's version. Your own customers may also ask you to defend them, so look at what you promised them.
Read the papers like a checklist
Which patent. Write down every patent number named. Get a copy of each patent, including the claims, the numbered sentences at the end. The claims define what the patent covers (35 U.S.C. 112(b)), not the title, the abstract, or the drawings.
Which claims. Infringement is judged claim by claim. Under the Supreme Court's decision in Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U.S. 17 (1997), each element of a claim is material to its scope, so a product must meet every element of a claim, literally or by an equivalent, to infringe that claim. A letter that names no claim, or names one without walking through its elements, tells you less than one that does.
Which product. Is the accused item yours, a component you buy from someone else, or something your customers do with your product? Does the description match what you actually make and sell, and the versions you sell now? Infringement under 35 U.S.C. 271(a) is making, using, offering to sell, or selling the patented invention in the United States, or importing it, during the patent's term, so where and when your product is sold matters too.
Whether a claim chart is attached. A claim chart lines up each element of a claim against features of the accused product. If one is attached, compare it with the real product, element by element, and note anything it gets wrong or skips. A complaint has to do more than accuse: it must contain a short and plain statement of the claim showing that the plaintiff is entitled to relief (Rule 8(a)(2)), and the Supreme Court has held that the facts alleged must make the claim plausible, allowing a reasonable inference that the defendant is liable (Ashcroft v. Iqbal, 556 U.S. 662 (2009)).
Whether the patent has expired. A utility patent's term generally ends 20 years from the date its application was filed in the United States or, if it claims the benefit of an earlier U.S. non-provisional or international application, from the earliest such filing date; a priority claim to a provisional or foreign application does not move that date earlier (35 U.S.C. 154(a)(2) and (a)(3)). Adjustments for USPTO delay, regulatory extensions, terminal disclaimers, and the older rule for applications filed before June 8, 1995 can all change that date (35 U.S.C. 154(b), 154(c)(1), 156, and 253(b)), so treat any estimate as a starting point. A utility patent also expires early if a maintenance fee is not paid on time or within the six-month grace period (35 U.S.C. 41(b)(2)), although the USPTO can accept a late payment if the delay was unintentional, and the patent is then treated as not having expired, with limited protection for anyone who began using the invention during the lapse (35 U.S.C. 41(c)). Design patents run 15 years from grant (35 U.S.C. 173); the USPTO notes that design patents from applications filed before May 13, 2015 have a 14-year term. The USPTO does not calculate expiration dates, but it offers a downloadable calculator to estimate them.
An expired patent can still support a claim for damages for the period before it expired. The limit runs the other way: no damages may be recovered for infringement committed more than six years before the complaint or counterclaim was filed (35 U.S.C. 286). The six years count back from the filing of a lawsuit, not from the date of a letter.
Read the rights holder's public record yourself
Much of what a patent owner has done with a patent is public, and you can read it yourself. Each item below is information, not a conclusion about anyone. A pattern in the record shows what happened, not why it happened.
Other suits on the same patent. Federal court dockets are on PACER, the judiciary's public access system, which has a Case Locator for searching by party name across courts. Look for other complaints that name the same patent number. Google Patents can show a litigation list on a patent's page, drawn from third-party data sets and covering the whole patent family, so treat it as a starting point and confirm each case on the docket. The record can tell you whether others received the same claim, which courts heard it, and whether a judge construed the claims or ruled on validity. It cannot tell you why particular defendants were chosen.
How those suits ended. Read the last entries on each docket. A plaintiff can dismiss its own case without a court order by filing a notice before the defendant serves an answer or a motion for summary judgment, or by a stipulation signed by all parties who have appeared, and unless the notice or stipulation says otherwise, the dismissal is without prejudice, meaning the claim can be brought again (Rule 41(a)(1)). A notice of dismissal does not have to say why the case ended. A series of early dismissals shows that cases ended early; a dismissal can follow a license, a settlement, a business decision, or something else, and the docket alone does not say which. Note how long each case lasted from complaint to dismissal, and look for orders on the merits, claim construction rulings, and any order on attorney fees.
Who owns the patent. The USPTO's Patent Assignment Search lets anyone search recorded patent assignments from 1980 to the present. Compare the recorded owner with the plaintiff. The Patent Act gives the remedy of a civil action for infringement to the patentee, a term that includes successors in title (35 U.S.C. 281 and 100(d)). Recording is not what makes a transfer valid between the parties; the statute makes an unrecorded assignment void only against a later purchaser or mortgagee who paid value without notice, unless the assignment was recorded within three months or before that later purchase (35 U.S.C. 261). So a mismatch can reflect an unrecorded transfer, a license, a name change, a merger, or a gap in the record. Whether a plaintiff has the right to sue is a question a court decides, and it is one a lawyer will look at.
Patent Office challenges. The Patent Trial and Appeal Board's case system, P-TACTS, is where the USPTO directs the public to search for inter partes reviews. Look for earlier petitions against the patent, whether review was instituted, and any final written decision. After a final written decision, once the time for appeal has passed or any appeal has ended, the USPTO issues a certificate canceling claims finally found unpatentable and confirming claims found patentable (35 U.S.C. 318(b)). A denied petition means review did not go forward; it is not a decision that the claims are valid. A claim that survived a review can still be challenged by others, although the petitioner in a review that reached a final written decision is barred from raising grounds it raised or reasonably could have raised (35 U.S.C. 315(e)).
Our sister site, Patent Litigator (patentlitigator.com), follows multi-defendant patent campaigns in the federal courts and at the ITC. Its coverage, like this page, is built from the public record.
The real options
Respond and ask for specifics. A measured written response, usually through counsel, can ask which claims are asserted, which products are accused, and for a claim chart for each. It can narrow the dispute to the claims and products actually at issue.
Analyze non-infringement and invalidity. Non-infringement compares each asserted claim to your product, element by element. Invalidity asks whether the claim should have issued at all: whether it covers patent-eligible subject matter (35 U.S.C. 101), whether it was new and non-obvious over earlier patents, publications, and products (35 U.S.C. 102 and 103), and whether the patent describes and claims the invention adequately (35 U.S.C. 112). In court, a patent is presumed valid and the challenger bears the burden (35 U.S.C. 282(a)), and the Supreme Court has held that invalidity must be proved by clear and convincing evidence (Microsoft Corp. v. i4i Ltd. Partnership, 564 U.S. 91 (2011)).
Early motions. Once you are sued, some defenses can be raised by motion before you answer. A motion under Rule 12(b)(6) argues that the complaint fails to state a claim. That can include a challenge under 35 U.S.C. 101, which courts analyze under the two-step framework of Alice Corp. v. CLS Bank International, 573 U.S. 208 (2014). The Federal Circuit has held that eligibility can be decided at the Rule 12(b)(6) stage, but only when no factual allegations, taken as true, prevent resolving it as a matter of law (Aatrix Software, Inc. v. Green Shades Software, Inc., 882 F.3d 1121 (Fed. Cir. 2018)). Venue is another early question. Under 28 U.S.C. 1400(b), a patent case may be brought where the defendant resides, or where it has committed acts of infringement and has a regular and established place of business, and in TC Heartland LLC v. Kraft Foods Group Brands LLC, 581 U.S. 258 (2017), the Supreme Court held that a domestic corporation resides only in its state of incorporation for this purpose. The Court did not address foreign companies. Objections to venue and personal jurisdiction are waived if they are not raised in your first Rule 12 motion or, if you make no motion, in your answer (Rule 12(h)(1)), so they have to be considered before anything is filed.
Inter partes review. Anyone who does not own the patent can petition the Patent Trial and Appeal Board to cancel claims as unpatentable, but only under sections 102 and 103 and only based on prior art patents and printed publications (35 U.S.C. 311(a) and (b)). The petitioner must prove unpatentability by a preponderance of the evidence (35 U.S.C. 316(e)), a lower standard than in court. Review is not automatic: the Director may institute only if the petition shows "a reasonable likelihood that the petitioner would prevail" on at least one claim, and the decision whether to institute is "final and nonappealable" (35 U.S.C. 314(a) and (d)). Timing matters: review may not be instituted on a petition filed more than one year after the petitioner, or its real party in interest or privy, is served with a complaint alleging infringement of the patent (35 U.S.C. 315(b)). The rules here are in motion. In October 2025 the USPTO proposed rules (90 Fed. Reg. 48335) that would, among other things, require a petitioner to stipulate that if review is instituted it will not raise section 102 or 103 invalidity grounds in any other proceeding, and would bar review of claims already upheld in certain earlier proceedings. Check the current rules and USPTO guidance before you plan around this option.
Declaratory judgment. You do not always have to wait to be sued. In a case of actual controversy, a federal court may declare the rights of an interested party, for example that a product does not infringe or that a patent is invalid (28 U.S.C. 2201(a)). The Supreme Court asks whether the facts alleged, under all the circumstances, show "a substantial controversy, between parties having adverse legal interests, of sufficient immediacy and reality" to warrant a declaratory judgment (MedImmune, Inc. v. Genentech, Inc., 549 U.S. 118 (2007)). A demand letter can be part of that picture, and the court decides. One interaction to know: filing your own lawsuit challenging the patent's validity before filing an inter partes review petition bars that review (35 U.S.C. 315(a)(1)), although a counterclaim of invalidity in a case the patent owner brought does not count (35 U.S.C. 315(a)(3)).
Settlement or a license. A dispute can also end in an agreement. If you go this way, read the terms closely: which products, customers, and suppliers the release covers; whether it covers past sales, future sales, or both; whether it covers related patents in the same family; what you are or are not admitting; whether the case is dismissed with or without prejudice; and whether a supplier's indemnity applies. A settlement is a business decision, and a sound one can be a good result.
Fee shifting. Under 35 U.S.C. 285, "the court in exceptional cases may award reasonable attorney fees to the prevailing party." In Octane Fitness, LLC v. ICON Health & Fitness, Inc., 572 U.S. 545 (2014), the Supreme Court held that an exceptional case "is simply one that stands out from others with respect to the substantive strength of a party's litigating position (considering both the governing law and the facts of the case) or the unreasonable manner in which the case was litigated," decided case by case in the district court's discretion, considering the totality of the circumstances. Fees are a possibility the court decides, never a promise. You must also be the prevailing party: in O.F. Mossberg & Sons, Inc. v. Timney Triggers, LLC, 955 F.3d 990 (Fed. Cir. 2020), the Federal Circuit held that a defendant was not a prevailing party after the plaintiff voluntarily dismissed its case without prejudice under Rule 41(a)(1)(A)(i), because there was no final court decision, even though the defendant had already persuaded the USPTO to invalidate the patent.
Fighting on principle is a legitimate choice
Some business owners would rather pay their own lawyer to defend a claim than pay a patent owner they believe is not entitled to anything. That is a legitimate choice. It deserves a clear view of what it involves.
Time. A patent case can move through pleadings, infringement and invalidity contentions, document discovery, depositions, claim construction, expert reports, summary judgment, trial, and appeal. A single stage can take months. Attention. You and your team will gather documents, answer written questions under oath, explain your product to your lawyers and experts, and sit for depositions, while still running the business. Cost. The cost is real and it continues for as long as the case does. In the United States, each side generally pays its own lawyers, win or lose, unless a statute or contract provides otherwise (Baker Botts L.L.P. v. ASARCO LLC, 576 U.S. 121 (2015)), and fee shifting under section 285 is reserved for exceptional cases, at the court's discretion.
Know what winning looks like before you start. A successful defense can end with the case dismissed and no money paid to you at all, not even your legal fees. If the plaintiff walks away by voluntary dismissal without prejudice, you may not even be a prevailing party. Decide at the outset what you are willing to spend and what you want to achieve, and revisit that decision at each stage. No one can promise an outcome, and a good result can look like many things: a dismissal, a narrower case, a license on terms you can live with, a design change, a supplier who steps in under an indemnity, or a ruling in your favor.
If you want a lawyer to look at it
Messages to this office are read personally, but the office cannot take every matter. It is a good fit for people who want a clear-eyed look at their situation and who understand that no lawyer can promise a result. If that sounds like you, use the contact form on the Contact page, linked at the top of every page on this site.
Before a lawyer can discuss your matter, the office must run a conflict check to confirm it does not represent the patent owner or anyone else involved. That check comes first, and it is why a useful first message includes only these facts: your name and your company's name; the names of the patent owner, its lawyers, and any supplier or customer involved; the case number and court, or the date of the letter; the patent number or numbers; the accused product, described in general terms; and any deadline, including the date you were served. The form asks you to send only what is needed to check for conflicts, so stick to those facts. Do not send the letter itself or other documents, your own analysis or strategy, or confidential details until the office confirms it can take the matter. Once the conflict check clears, the first conversation is the place to talk about your goals, including whether you would rather resolve the matter or contest it.
A message does not create an attorney-client relationship, and until an engagement is confirmed in writing, you are not a client. If your deadline is days away, say so in the first line, and do not let a pending message stand in for protecting that deadline.