What is interference practice in patent law?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

Interference practice is a procedure used to determine priority of invention between two parties. It is based on pre-AIA 35 U.S.C. 102(g). As stated in the MPEP:

“An interference is an inter partes proceeding directed at determining the first to invent as among the parties to the proceeding, involving two or more pending applications naming different inventors or one or more pending applications and one or more unexpired patents naming different inventors.”

This process is unique to the U.S. patent system, which has historically operated on a first-to-invent basis rather than a first-to-file system for certain applications.