Refers to a patent office proceeding to re-examine the grant of a patent, usually in light of prior art not before the patent office in the original examination. There are two types of re-examination, Ex Parte and Inter Partes. Despite the view sometimes expressed in the media that a reexamined patent is somehow flawed, or of questionable validity, in fact a patent that has survived reexamination is considerably strengthened, and usually much more difficult to challenge on the basis of the art considered in the reexamination.

While reexamination does not exist in the European Patent Office, there is a somewhat analogous procedure known as opposition. In principle, the Director of the USPTO can also order a reexamination, but this is a very unusual procedure, usually only applied to controversial or joke patents, although there are anecdotal suggestions that political lobbying gave rise to such reexaminations in high-profile patent cases.

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