Intereferences will be held for applications and patents having an effective filing date before March 16, 2013.  Applications filed after this date are not eligible for interference proceedings. 

Interferences may occur between a pending application and one or more other pending applications or a pending application and one or more unexpired patents. Interferences are held when the filing dates are so close together that there is a possibility that the first to file for particular claims may not be the first inventor. They only apply to applications filed before the first to invent system switched to the first to file system.

There are two types of interferences; those between two applications and those between an application and a patent.   Interferences between two applications are normally instigated by the examiner. If there is only a difference of 6 months between the filing dates of two applications that claim identical subject matter, an examiner will provoke an interference. Interferences involving an application and a patent are normally provoked by the applicant. In these instances, the patent usually only recently issued so the applicant never found it in his or her examination search.

The interfering subject matter or claims between the two applications or the application and the patent are referred to as the count. The two opposing parties in an interference attempt to show that they first reduced the invention to practice.

Whoever can prove that they made the invention workable first usually wins in an interference.

The winning party of the interference is not denied anything he or she was in possession of prior to the interference, nor does he or she acquire any additional rights as a result of the interference. The winner just gets to either continue along with their examination (if it was an application) or live out the patent term (if it was a patent). The loser of an interference may appeal to the Court of Appeals for the Federal Circuit or file a civil action in a U.S. district court.

Interferences are replaced by Derivation Proceedings as of March 16, 2013 as outlined in the America Invents Act (AIA). The AIA provides for derivation proceedings and eliminates the interference practice for applications and patents having an effective filing date on or after March 16, 2013 (with a few exceptions).