Patent infringement cases are federal civil lawsuits brought by a patent holder against someone who makes, uses, sells, offers to sell, or imports a patented invention without permission. A utility patent runs for 20 years from its filing date, and during that window the owner can sue to stop the unauthorized use and recover money.1Office of the Law Revision Counsel. 35 USC 154 – Contents and Term of Patent These suits can only be heard in U.S. district courts, they often last years, and the money at stake routinely reaches the millions. What follows is what matters on both sides of the dispute: what counts as infringement, how courts decide it, what defenses work, and what a win is actually worth.
What Counts as Infringement
Direct Infringement
Direct infringement is the core claim. Anyone who makes, uses, offers to sell, sells, or imports a patented invention in the United States without authorization is a direct infringer.2Office of the Law Revision Counsel. 35 USC 271 – Infringement of Patent Intent is irrelevant. This is a strict liability claim: the patent holder only has to show the accused product or process falls within the scope of a valid claim. Even a company that developed the same technology independently, having never heard of the patent, can be liable.
Induced and Contributory Infringement
Indirect infringement reaches parties who enable someone else’s infringement. Inducement means actively encouraging another to infringe while knowing the patent exists and that the encouraged conduct will infringe.3Legal Information Institute. Inducement of Infringement Contributory infringement means selling a component that is a material part of a patented invention, knowing it was specially made for infringing use, where the component has no other significant legitimate use.2Office of the Law Revision Counsel. 35 USC 271 – Infringement of Patent Both require proof the defendant knew about the patent, which sets them apart from direct infringement.
Willful Infringement
Willfulness is not a separate claim. It is a finding that can triple the damages award under 35 U.S.C. § 284.4Office of the Law Revision Counsel. 35 USC 284 – Damages The Supreme Court’s 2016 decision in Halo Electronics, Inc. v. Pulse Electronics, Inc. made willfulness easier to prove, replacing a rigid two-part test with a flexible one and lowering the burden of proof from “clear and convincing evidence” to a preponderance of the evidence. A defendant who knew about the patent and had no reasonable basis for believing it was clear faces real exposure to enhanced damages.
How Courts Decide If a Product Infringes
Claim Construction
Before anyone compares a patent to an accused product, the judge has to decide what the patent’s claims mean. That happens at a claim construction hearing, often called a Markman hearing. The judge looks at the patent’s written description, the prosecution history between the inventor and the Patent Office, and sometimes expert testimony to fix the meaning of the key technical terms. This phase often decides the case. How a term is defined can take infringement off the table entirely, or hand it to the plaintiff, well before trial.
Literal Infringement
Once the terms are defined, literal infringement is a straight comparison. If every element listed in the patent claim appears in the accused product, there is infringement. If a single element is missing, literal infringement fails. The all-elements rule is strict, and it gives defendants a clean benchmark for how much exposure they have.
Doctrine of Equivalents
When literal infringement doesn’t fit, the patent holder can still prevail under the doctrine of equivalents. The accused product infringes if it performs the same function, in the same way, to achieve the same result as the patented invention, even if the details differ slightly.5United States Patent and Trademark Office. MPEP 2186 – Relationship to the Doctrine of Equivalents The doctrine blocks competitors from making trivial changes to escape the literal claim language while copying the core idea.
There is a significant limit. Under prosecution history estoppel, if an applicant narrowed a claim during examination to get the patent granted, the applicant generally cannot use the doctrine of equivalents to recapture what was given up. Ground surrendered to the Patent Office stays surrendered.
Filing a Patent Infringement Lawsuit
Where the Case Can Be Filed
A patent suit can only be filed in a U.S. district court where the defendant resides or has committed acts of infringement and has a regular and established place of business.6Office of the Law Revision Counsel. 28 USC 1400 – Patents and Copyrights, Mask Works, and Designs For a domestic corporation, “resides” means the state of incorporation. The Supreme Court confirmed that reading in TC Heartland LLC v. Kraft Foods Group Brands LLC, and the ruling sharply curtailed forum shopping into plaintiff-friendly districts where the defendant had only a thin connection.7Supreme Court of the United States. TC Heartland LLC v Kraft Foods Group Brands LLC
What the Complaint Must Say
The complaint has to identify the patent, the specific claims believed to be infringed, the accused product or process, and the basis for the court’s jurisdiction. Since the former model patent complaint (Form 18) was abolished in 2015, patent complaints must meet the general federal pleading standard: enough factual detail to make the infringement claim plausible on its face, not just possible.
Serving the Defendant
After filing, the plaintiff must formally deliver the summons and complaint to the defendant. Federal Rule of Civil Procedure 4 spells out the methods, which depend on whether the defendant is an individual, a corporation, or located outside the United States.8Legal Information Institute. Federal Rules of Civil Procedure Rule 4 – Summons Proof of service goes into the court file before the case moves forward. A defendant can waive formal service, which saves the plaintiff some expense and gives the defendant more time to respond.
Defenses That Can Defeat a Patent Case
Defendants rarely stop at arguing they don’t infringe. The stronger move is often to attack the patent itself.
Invalidity Based on Prior Art
A patent is invalid if the invention was already known before the application was filed. Under the anticipation defense, a defendant points to a single prior publication, patent, or public use that disclosed every element of the claim before the patent’s effective filing date.9Office of the Law Revision Counsel. 35 USC 102 – Conditions for Patentability; Novelty If no single reference covers everything, the defendant can argue obviousness: that a person with ordinary skill in the field would have found the invention obvious by combining what was already out there. Courts weigh the scope of the prior art, the differences from the claim, the level of skill in the field, and objective evidence such as commercial success or long-unmet need.
Prior Commercial Use
A defendant who was commercially using the technology before the patent was filed has a personal defense under 35 U.S.C. § 273. The commercial use must have started at least one year before the patent’s effective filing date, and the defendant must prove it by clear and convincing evidence.10Office of the Law Revision Counsel. 35 USC 273 – Defense to Infringement Based on Prior Commercial Use The defense is personal to the party who did the using, and it generally cannot be licensed or transferred except as part of a good-faith sale of an entire business line. If the defense is asserted without a reasonable basis, the court can treat the case as exceptional and award attorney fees to the patent holder.
Inequitable Conduct
If the patent applicant withheld important information from the Patent Office or made material misrepresentations during examination, a court can refuse to enforce the entire patent. The defense requires two findings: that the information was material to patentability, and that the applicant acted with specific intent to deceive. Intent can be inferred from circumstantial evidence, but materiality alone is not enough. When proven, inequitable conduct makes the patent unenforceable even if infringement occurred.
Marking and the Six-Year Damages Cap
Patent Marking
Patent holders who sell a patented product should mark it with the patent number or a web address linking the product to the patent. Skipping that step has a direct cost: the patent holder cannot collect damages for infringement that happened before the infringer received actual notice.11Office of the Law Revision Counsel. 35 USC 287 – Limitation on Damages and Other Remedies; Marking and Notice Filing the lawsuit itself counts as notice, but any infringement before the filing date is unrecoverable without proof of earlier notification. Proper marking from day one preserves the full damages window.
The Six-Year Lookback
There is no hard deadline for filing suit during the life of the patent, but damages are capped at infringement that occurred within six years before the complaint was filed.12Office of the Law Revision Counsel. 35 USC 286 – Time Limitation on Damages If a competitor has been infringing for eight years and you file today, you recover for the most recent six. Waiting too long also opens the door to a laches defense based on prejudice from the delay.
Challenging a Patent at the PTAB
Not every dispute belongs in district court. The Patent Trial and Appeal Board at the USPTO handles faster, cheaper proceedings to challenge a patent’s validity, and they often run alongside the federal court case.
Inter Partes Review
Inter partes review lets a third party challenge one or more claims based on prior patents or printed publications. The petitioner must show a reasonable likelihood of prevailing on at least one claim. For patents under the first-inventor-to-file system, a petition cannot be filed until nine months after the patent is granted.13United States Patent and Trademark Office. Inter Partes Review Once instituted, the Board must issue a final decision within one year, with a possible six-month extension for good cause. IPR has become one of the most common tools for patent defendants.
Post-Grant Review
Post-grant review is broader. It allows challenges on any ground of invalidity, not just prior art. The window is tighter: the petition must be filed within nine months of the patent’s grant. The petitioner must also meet a higher threshold, showing it is more likely than not that at least one challenged claim is unpatentable.14United States Patent and Trademark Office. Post Grant Review PGR is only available for patents filed under the America Invents Act’s first-inventor-to-file provisions.
What a Win Is Worth
Monetary Damages
A prevailing patent holder is entitled to damages adequate to compensate for the infringement, with a floor of a reasonable royalty.4Office of the Law Revision Counsel. 35 USC 284 – Damages A reasonable royalty is what the infringer would have agreed to pay in a hypothetical licensing negotiation before infringement began. Where the patent holder can show it lost sales it otherwise would have made, lost profits are available instead and usually produce a larger number. The court adds interest and costs.
For egregious infringement, the court can increase the award up to three times the compensatory amount.4Office of the Law Revision Counsel. 35 USC 284 – Damages Enhanced damages are not automatic even after a willfulness finding. The judge weighs how deliberate the infringement was, how the infringer behaved during litigation, and whether it had any good-faith basis for its conduct.
Injunctions
Courts can issue a permanent injunction ordering the infringer to stop making, using, or selling the invention for the rest of the patent term.15Office of the Law Revision Counsel. 35 USC 283 – Injunction Injunctions are no longer automatic. Since the Supreme Court’s 2006 decision in eBay Inc. v. MercExchange, L.L.C., the patent holder must show irreparable injury, that money damages are inadequate, that the balance of hardships tips in favor of an injunction, and that the public interest would not be harmed.16Library of Congress. eBay Inc v MercExchange LLC, 547 US 388 The standard has made injunctions harder to get, especially for patent holders who license rather than manufacture, because they struggle to prove harm that money cannot fix.
Attorney Fees
In exceptional cases, the court can award reasonable attorney fees to the prevailing party, whether that is the patent holder or the accused infringer.17Office of the Law Revision Counsel. 35 USC 285 – Attorney Fees A case is exceptional when it stands out from the norm based on the strength of the losing party’s position or the way the case was litigated. Fee-shifting works both ways. A patent holder bringing a frivolous case can end up paying the defendant’s legal bills, and a defendant that infringes egregiously and stonewalls through litigation can end up paying the patent holder’s fees on top of damages.
Appeals Go Only to the Federal Circuit
Every patent appeal from every district court in the country goes to one place: the U.S. Court of Appeals for the Federal Circuit, which has exclusive jurisdiction over patent appeals.18Office of the Law Revision Counsel. 28 USC 1295 – Jurisdiction of the United States Court of Appeals for the Federal Circuit The point of that centralization is uniformity. Without it, regional circuits could develop conflicting rules on infringement, validity, and damages. The Federal Circuit reviews claim construction, jury verdicts, damages, and injunction decisions under different standards depending on the issue. A further appeal to the Supreme Court is possible but rarely granted.
What Patent Litigation Costs
Patent cases are among the most expensive civil lawsuits in federal court. Surveys by the American Intellectual Property Law Association put the median cost of taking a patent case through trial at roughly $600,000 when less than $1 million is at stake, rising past $5 million in the highest-stakes disputes. Discovery drives most of that spend, through technical experts, document review, and claim construction. Expert witnesses commonly charge $450 to $600 per hour, and specialized patent litigators bill at rates that swing widely by market and by how complex the technology is.
Those numbers explain why most patent disputes settle and why PTAB proceedings have become popular alternatives. An inter partes review is substantially cheaper than full litigation, though it only addresses validity and does not produce damages. For any patent holder weighing whether to enforce, the first calculation is a realistic comparison between litigation cost and likely recovery.