What is inequitable conduct in patent prosecution?

Inequitable conduct, often called “fraud upon the patent office,” generally means that in trying to obtain a patent, the applicant was not honest with the patent office and failed to disclose prior art or other information known by the applicant that was material to the patentability of his/her invention.  Patent prosecution rules include a Duty of Candor, which requires the applicant and his/her representatives to disclose material information to the USPTO during prosecution of the patent application.  Should an applicant withhold material information and thus commit inequitable conduct, the result is that the patent would be deemed unenforceable.  This is an equitable doctrine.  It means that even if the patent is valid, the applicant would be precluded from enforcing it against infringers due to the fraud upon the patent office.  Inequitable conduct is a defense sometimes raised to allegations of infringement.