In law, a patent infringement occurs when the subject-matter claimed in a patent has been utilized by someone other than the rightholder, without the owner`s approval or in disagreement with the terms of use given by the owner. Depending on the patent laws in the country where the patent infringement has taken place, the owner of the patent may take action in equity or in law such as an injunction or lawsuit against those who did the infringement. In U.S. law, an infringement may occur where the defendant has made, used, sold, offered to sell, or imported the infringing invention or its equivalent. No infringement action may be started until the patent is issued.

Defenses

The single most common defense to patent infringement is a counter-attack on the patent itself, i.e., the validity of the patent and the allegedly infringed claims. Even if the patent is valid, the plaintiff must still prove that every element of at least one claim was infringed and that such infringement caused some sort of damage. In case of a medical procedure patent issued after 1996, a U.S. infringer may also raise a statutory safe harbor defense to infringement.

Contributory infringement

Under certain jurisdictions, there is a particular case of patent infringement, called ``contributory infringement``, ``indirect infringement`` or ``induced infringement``. This can occur for instance when a device is claimed in a patent and when a third party supplies a product which can only be reasonably used to make the claimed device.

In the United States, USC 35 ? 271(b) defines (active) induced infringement: ``Whoever actively induces infringement of a patent shall be liable as an infringer.``

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http://en.wikipedia.org/wiki/Patent_infringement