If you have found prior art that you believe may render the subject matter of an already issued patent unpatentable, you may file a reexamination.

For a reexamination, the evidence is limited to patents and printed publications only. The America Invents Act (AIA) also expanded on this to include written statements of a patent owner filed in a proceeding before a Federal court or the USPTO regarding the scope of any claim of the patent. 

The grounds of a reexamination are limited to issues concerning the patentability of the subject matter. Issues of fraud and inequitable conduct are never appropriate matters for a reexamination.

Anyone may request a reexamination; any member of the public, including the patent owner or even corporate or government entities. However, if a reexamination is initiated by anyone other than the patent owner, the reexamination will need to be serviced to the owner. The person requesting the reexamination may opt to keep his or her identity secret.

Reexaminations may be requested at any time during the period of enforceability provided a substantial new question of patentability is presented. The period of enforceability includes the patent's full term plus another 6 years after the patent expires.

Reexamination requests must include a copy of every publication, written statement by the patent owner, or patent that brings up a substantial new question of patentability (SNQ). This means that the prior art already used by the examiner during the prosecution may not be cited in the reexamination proceeding.  It must be new. In addition, the person filing the reexamination must include a fee along with the appropriate paperwork.