The extent to which a trademark owner may prevent unauthorized use of trademarks that are the same as or similar to its trademark depends on various factors such as whether its trademark is registered, the similarity of the trademarks involved, the similarity of the products or services involved, and whether the owner's trademark is well known or, under U.S. law relating to trademark dilution, famous.

If a trademark has not been registered, some jurisdictions (especially Common Law countries) offer protection for the business reputation or goodwill which attaches to unregistered trademarks through the tort of passing off. Passing off may provide a remedy in a scenario where a business has been trading under an unregistered trademark for many years, and a rival business starts using the same or a similar mark.

If a trademark has been registered, then it is much easier for the trademark owner to demonstrate its trademark rights and to enforce these rights through an infringement action. Unauthorized use of a registered trademark need not be intentional for infringement to occur, although damages in an infringement lawsuit will generally be greater if there was an intention to deceive.

For trademarks that are considered to be well known, infringing use may occur where the use occurs about products or services which are not the same as or similar to the products or services about which the owner's mark is registered. A growing area of law relating to the enforcement of trademark rights is secondary liability, which allows for the imputation of liability to one who has not acted directly to infringe a trademark but whose legal responsibility may arise under the doctrines of either contributory or vicarious liability.

Limits and defenses to claims of infringement.

Trademarks are subject to various defenses, such as abandonment, limitations on geographic scope, and fair use. In the United States, the fair use defense protects many of the interests in free expression related to those protected by the First Amendment.

Fair use may be asserted on two grounds, either that the alleged infringer is using the mark to describe accurately an aspect of its products, or that the alleged infringer is using the mark to identify the mark owner. One of the most visible proofs that trademarks provide a limited right in the U.S. comes from the comparative advertising that is seen throughout U.S. media.

An example of the first type is that although Maytag owns the trademark "Whisper Quiet" for its dishwashers, makers of other products may describe their goods as being "whisper quiet" so long as these products do not fall under the same category of goods the trademark is protected under.

An example of the second type is that Audi can run advertisements saying that a trade publication has rated an Audi model higher than a BMW model since they are only using "BMW" to identify the competitor. In a related sense, an auto mechanic can truthfully advertise that he services Volkswagens, and a former Playboy Playmate of the Year can identify herself as such on her website.

Wrongful or groundless threats of infringement.

Various jurisdictions have laws that are designed to prevent trademark owners from making wrongful threats of a trademark infringement action against other parties. These laws are intended to prevent large or powerful companies from intimidating or harassing smaller companies.

Where one party makes a threat to sue another for trademark infringement, but does not have a genuine basis or intention to carry out that threat, or does not carry out the threat at all within a certain period, the threat may itself become a basis for legal action. In this situation, the party receiving such a threat may seek from the Court a declaratory judgment; also known as a declaratory ruling.