What Is Patent Infringement — and What Can You Do About It?

What Is Patent Infringement? The Legal Definition

Under 35 U.S.C. § 271, patent infringement occurs when a party, without authorization, makes, uses, offers to sell, sells, or imports into the United States any patented invention during the term of the patent. The key word is "claims" — infringement is determined by comparing the accused product or process to the language of the patent's claims, not to the invention as described in the specification or drawings.

This is why claim drafting is so consequential. The claims are the legal boundaries of your patent rights. A narrow claim provides narrow protection. A broad claim, if it survives examination, provides broad protection — but is also more vulnerable to invalidity challenges. Understanding what your claims actually cover is the starting point for any infringement analysis.

Types of Patent Infringement: Direct, Indirect, and Contributory

Not all infringement looks the same. The law recognizes several distinct forms:

Direct infringement is the most straightforward: a party performs every element of at least one patent claim. The infringer need not know the patent exists. Intent is irrelevant to liability for direct infringement, though it matters significantly for the damages calculation.

Induced infringement (indirect) occurs when one party actively encourages, instructs, or induces another to directly infringe a patent. A company that publishes instructions telling its customers how to use a product in a way that infringes a claim, knowing of the patent, can be liable for inducement even if the company itself is not performing the infringing act. Crucially, inducement requires actual knowledge of the patent and intent to cause infringement.

Contributory infringement occurs when a party sells or imports a component of a patented invention, knowing that the component is especially designed for use in an infringement and has no substantial non-infringing use. Selling a specialized part with no legitimate purpose outside a patented machine, with knowledge of the patent, is the paradigm case.

There is also infringement under the doctrine of equivalents: even if an accused product does not literally satisfy every claim element, infringement may still be found if each element is substantially the same in function, way, and result as the corresponding claim element. This doctrine prevents competitors from making trivial design-arounds that capture the essence of the invention without technically falling within the literal claim language.

How to Determine Whether Your Patent Is Being Infringed

Infringement analysis is a two-step process. First, the claims are construed — their meaning and scope are determined as a matter of law, guided by the claim language, the specification, the prosecution history, and relevant extrinsic evidence. Second, the accused product or process is compared element-by-element to the construed claims.

For every independent claim you assert, every single element of that claim must be present in the accused product or process — either literally or under the doctrine of equivalents. If even one element is absent, there is no infringement of that claim. This is called the "all-elements rule."

Conducting this analysis properly requires both legal expertise and technical knowledge. A patent attorney familiar with your technology can review publicly available product specifications, patent disclosures (competitors are required to disclose their own patents), marketing materials, and reverse-engineering reports to build an infringement read. If you believe a competitor is infringing, start by collecting as much publicly available information about their product or process as possible before engaging counsel.

Cease-and-Desist Letters: When and How to Use Them

A cease-and-desist (C&D) letter is often the first formal step in patent enforcement. The letter notifies the alleged infringer of the patent, identifies the asserted claims, explains why the patent holder believes infringement is occurring, and demands that the infringer stop. It may also provide the alleged infringer with an opportunity to take a license.

However, C&D letters carry risks that non-attorneys frequently overlook. In some cases, a poorly drafted or premature C&D can trigger the recipient to file a declaratory judgment action — essentially a lawsuit asking a court to declare the patent invalid or not infringed — in a jurisdiction of the accused infringer's choosing rather than yours. The letter can also harden negotiations if sent without a clear enforcement strategy behind it.

Before sending any patent-related cease-and-desist correspondence, consult with a registered patent attorney. The letter needs to be accurate about what the patent covers, credible in its enforcement posture, and part of a broader strategy — whether that strategy is licensing, litigation, or negotiated settlement.

Licensing as an Alternative to Litigation

Litigation is expensive, slow, and uncertain. For most patent disputes — especially those involving small or medium-sized businesses — a negotiated license is a faster and more economical path to value. A patent license allows the infringer to continue operating while paying the patent holder royalties or a lump-sum fee. Both parties avoid the costs and risks of litigation.

Licensing negotiations work best when the patent's validity and infringement read are strong, the infringer has a viable business that is worth licensing rather than shutting down, and both parties have an interest in resolving the matter commercially. An attorney can help you structure licensing terms — royalty rates, field-of-use limitations, sublicensing rights, and audit provisions — that protect your long-term IP value while achieving near-term commercial goals. See our IP strategy services for more on monetization approaches.

Patent Litigation: Costs, Timelines, and What to Expect

When licensing negotiations fail, patent litigation is the enforcement mechanism of last resort. Patent cases are filed in federal district courts, and the American Intellectual Property Law Association (AIPLA) surveys routinely report that patent litigation through trial can cost between $1.5 million and $4 million or more per side, depending on complexity. For smaller disputes, costs are lower, but still substantial.

Timeline from filing to trial typically runs 18 months to 3 years, with claim construction (Markman) hearings, fact discovery, expert discovery, summary judgment, and pre-trial motions preceding any trial. Many cases settle before trial — often after claim construction, when both sides have a clearer view of the patent's actual scope and the strength of the invalidity defenses.

The Inter Partes Review (IPR) process at the USPTO Patent Trial and Appeal Board (PTAB) provides an alternative forum where patent validity can be challenged on prior art grounds, typically faster and cheaper than district court. Any serious infringement enforcement strategy must account for the likelihood that the accused infringer will seek IPR review of the asserted patent.

What to Do If You Receive an Infringement Notice

If you receive a cease-and-desist letter or infringement notice, do not ignore it and do not respond without legal counsel. Several time-sensitive considerations come into play:

• Evaluate the patent. Is it valid? Does it actually cover what you are doing? A freedom-to-operate (FTO) analysis and invalidity search are your first tools.
• Assess the claim read. Work with your attorney to perform an element-by-element comparison of the asserted claims to your product or process. There may be non-infringement arguments the patentee has overlooked or overstated.
• Consider design-arounds. If infringement appears likely, can your product or process be modified to avoid the patent claims while maintaining commercial function? A design-around may be faster and cheaper than litigation.
• Investigate IPR options. If the patent appears vulnerable on prior art grounds, an IPR petition at the PTAB may be an effective tool to invalidate the patent efficiently.
• Do not destroy documents. Once you have notice of potential litigation, litigation hold obligations may apply. Destroying documents after receiving an infringement notice can result in severe sanctions.

The Importance of Patent Marking

Patent marking — placing the patent number (or a URL to a webpage listing the patent) on your patented product — is important for enforcement. Under 35 U.S.C. § 287, if you fail to mark your patented products, you may be unable to recover damages for infringement that occurred before you gave the infringer actual notice of the patent. In practice, this can significantly reduce the damages available in litigation. Marking costs nothing and preserves your full damages window from the moment the product enters the market. If you have an issued patent that covers a physical product, marking should begin immediately.