To prove patent infringement, you have to show by a preponderance of the evidence that an accused product or process contains every element of at least one claim in your patent, and that it was made, used, sold, offered for sale, or imported in the United States without your authorization during the patent term.1Office of the Law Revision Counsel. 35 USC 271 – Infringement of Patent “Preponderance” means more likely than not. Everything else in a patent case, from discovery strategy to expert reports to trial, is built around that element-by-element comparison.
What You Are Actually Proving
Infringement lives in the claims. Those are the numbered sentences at the end of the patent document, and they define the exact boundary of what the patent protects. The drawings, the written description, and the abstract exist to support the claims, but they are not what an accused product is measured against.
Each independent claim stands on its own. If the accused product contains every element of any single independent claim, that is enough to establish infringement of that claim. Dependent claims add further limitations and narrow the scope; a product that infringes a dependent claim necessarily infringes the independent claim it depends on, but the reverse is not true. Your proof is organized claim by claim and, within each claim, element by element. Miss one element and that claim fails.
The Two Paths: Literal Infringement and Equivalents
Literal Infringement
The cleanest proof is literal infringement: showing that the accused product contains every element of a claim, exactly as the claim describes it. This is usually documented in a claim chart, a two-column document that places each limitation from the claim next to the specific feature in the accused product that satisfies it.2United States Patent and Trademark Office. Manual of Patent Examining Procedure 2186 – Relationship to the Doctrine of Equivalents If one row of that chart is blank, literal infringement of that claim fails.
Whether a feature in the accused product actually “matches” a claim element is not a matter of plain reading. It depends on what the court has decided the claim language means. That is why claim construction, discussed below, drives so much of the case.
The Doctrine of Equivalents
When the accused product does not match a claim element word for word, you may still prove infringement under the doctrine of equivalents. You have to show that the accused product performs substantially the same function, in substantially the same way, to achieve substantially the same result as the patented invention.3Legal Information Institute. Doctrine of Equivalents The analysis is applied element by element, not to the invention as a whole; each missing element must independently pass the function-way-result test.2United States Patent and Trademark Office. Manual of Patent Examining Procedure 2186 – Relationship to the Doctrine of Equivalents
This path has a significant limit. If you narrowed a claim during the patent application process to get around prior art the examiner cited, you generally cannot use the doctrine of equivalents to recapture what you gave up. The Supreme Court held in Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co. that a narrowing amendment creates a presumption that you surrendered everything between the original and amended claim language.4Justia. Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 US 722 (2002) You can rebut that presumption only in narrow circumstances: the equivalent was unforeseeable at filing, the amendment was only tangentially related to the equivalent, or some other reason shows you could not reasonably have been expected to draft around it. Before relying on equivalents at trial, work through the prosecution history carefully.
Match Your Proof to the Type of Infringement
What you have to prove depends on which kind of infringement you are alleging.
Direct infringement is the simplest. Anyone who makes, uses, sells, offers to sell, or imports the patented invention in the United States during the patent term infringes, and intent is irrelevant. If the product falls within the claims, the act itself is enough.1Office of the Law Revision Counsel. 35 USC 271 – Infringement of Patent
Induced infringement reaches someone who does not infringe directly but actively encourages or instructs another party to do so. Here, you also have to prove the accused party knew about the patent and intended their actions to cause infringement. A company that distributes detailed instructions for using a product in an infringing way is the classic target.1Office of the Law Revision Counsel. 35 USC 271 – Infringement of Patent
Contributory infringement involves selling or importing a component specifically designed for use in an infringing product, with knowledge that it is built for that purpose. The component cannot be a common off-the-shelf item with legitimate non-infringing uses; it has to be something essentially made for infringement.1Office of the Law Revision Counsel. 35 USC 271 – Infringement of Patent
Collecting the Evidence
Before You File
You need a reasonable basis for your infringement claim before you file suit. The accused product itself is the best starting point. Buy it off the shelf, download the software, or obtain it through normal commercial channels so you can physically examine it and compare it against your claims. User manuals, spec sheets, marketing brochures, and product demonstrations fill in gaps about how the product works.
When internal workings are not visible from the outside, reverse engineering becomes essential. Disassembling the product, running diagnostic tests, and analyzing components reveals how it is built and whether it incorporates the patented technology. Document the process methodically. These findings become the backbone of the technical case and should map directly onto a claim chart.
Discovery
Once the lawsuit is filed, discovery opens up evidence you could not reach before. You can request internal design documents, engineering specifications, manufacturing processes, testing results, and communications about the accused product. Depositions let you question the defendant’s engineers and decision-makers under oath about how the product actually works.
Software cases bring a particular challenge, because source code is rarely public. Courts handle this through protective orders that let outside counsel and technical experts review the code under strict security conditions. Access is typically limited to viewing on a secure computer, with tight restrictions on printing and copying, and the defendant’s own employees may be barred from seeing the patent holder’s analysis to prevent competitive intelligence leaks.
Claim Construction Shapes the Proof
Before either side can argue whether the accused product infringes, the court has to decide what the claims actually mean. The Supreme Court held in Markman v. Westview Instruments that interpreting patent claims is a question of law for the judge, not the jury.5Justia. Markman v. Westview Instruments, Inc., 517 US 370 (1996)
That interpretation happens at a Markman hearing, usually before trial. Both sides present arguments about disputed claim terms. The judge looks at the claim language, the patent specification, and the prosecution history (the back-and-forth with the patent examiner during the application process). Expert testimony and technical dictionaries can supplement the analysis, but that outside evidence cannot override a meaning that is clear from the patent itself.
The claim construction order governs the rest of the case. A narrow reading of a key term can doom your infringement theory; a broad reading can expose the patent to invalidity challenges. Many cases settle immediately after this ruling because one side realizes the court’s definitions have made their position untenable. Build your evidence with claim construction in mind, and be prepared to adjust once the order issues.
Technical Experts
Most patent cases involve technology complex enough that a judge and jury cannot evaluate infringement without help. Technical experts walk the fact-finder through the claim chart, element by element, and explain whether each is present literally or under the doctrine of equivalents.
Federal courts apply the Daubert standard to screen expert testimony for reliability before the jury hears it. The judge evaluates whether the expert’s methodology can be tested, whether it has been peer-reviewed, its error rate, and whether it is accepted within the relevant field.6Legal Information Institute. Daubert Standard A weak expert report is one of the fastest ways to lose a patent case, because if the court excludes your expert, you may be left without the technical evidence you need to prove your claim at all. Both sides usually retain experts who reach opposite conclusions, and the battle comes down to whose methodology is more rigorous and whose explanation is more credible.
Defenses Your Proof Has to Survive
Proving every element of a claim is not the end of the story. The accused infringer will push back, and some defenses can neutralize an otherwise solid infringement case.
The most common response is not “we don’t infringe.” It is “your patent should never have been granted.” Every issued patent carries a legal presumption of validity, and an accused infringer has to show invalidity by clear and convincing evidence, a higher bar than the preponderance standard you face on infringement.7Office of the Law Revision Counsel. 35 US Code 282 – Presumption of Validity, Defenses Common invalidity grounds include prior art the examiner did not consider, arguments that the invention was obvious, and challenges to whether the specification adequately describes how to make and use the invention. If a claim is invalidated, it cannot be infringed, and your case on that claim is over.
A defendant can also petition the Patent Trial and Appeal Board at the USPTO for inter partes review. The Board can cancel claims based on prior art patents and publications, and the standard there is lower: preponderance of the evidence rather than clear and convincing evidence. If the Board cancels the claims you are asserting, your infringement case collapses. Final decisions are generally issued within 12 to 18 months of institution.8United States Patent and Trademark Office. Inter Partes Review
One last trap on the proof side: patent marking. If you or your licensees sell products covered by the patent, federal law requires those products be marked with the patent number. Failure to mark limits your damages to infringement that occurred after the infringer received actual notice of the patent. Virtual marking, where products carry a web address linking to the patent number, satisfies the requirement.9Office of the Law Revision Counsel. 35 USC 287 – Limitation on Damages and Other Remedies Proving infringement without a marking record intact can still win the case, but it shrinks the recovery.