Trademark infringement is the unauthorized use of a trademark or service mark (or a substantially similar mark) on competing or related goods and services. The success of a lawsuit to stop the infringement turns on whether the defendant's use causes a likelihood of confusion in the average consumer.

When infringement occurs, a trademark owner (the plaintiff) may file a lawsuit against the infringing user of the same or similar mark (the defendant) to prevent further use of the mark and collect money damages for the wrongful use. An infringement action may be brought in state court or in federal court if the mark in question is protected under the Lanham Act, which applies to both registered and unregistered marks that are used in commerce that Congress may regulate.

The success of an infringement normally turns on whether the defendant’s use causes a likelihood of confusion and so weakens the value of the plaintiff’s mark. A mark need not be identical to one already in use to infringe upon the owner’s rights. If the proposed mark is similar enough to the earlier mark to risk confusing the average consumer, its use may constitute infringement if the services or goods on which the two marks are used are related to each other—that is, they share the same market. For more on the standards of confusion see the articles and questions, below.