Prior art (also known as state of the art or background art) is a concept in patent law used to determine the patentability of an invention, in particular whether an invention meets the novelty and the inventive step or non-obviousness criteria for patentability. In most systems of patent law, prior art is generally defined as anything that is made available, or disclosed, to the public that might be relevant to a patent's claim before the effective filing date of a patent application for an invention. However, notable differences exist in how prior art is specifically defined under different national, regional, and international patent systems.

The prior art is evaluated by patent offices as part of the patent granting process in what is called "substantive examination" of a patent application in order to determine whether an invention claimed in the patent application meets the novelty and inventive step or non-obviousness criteria for patentability. It may also be considered by patent offices or courts in opposition or invalidity proceedings. Patents disclose to society how an invention is practiced, in return for the right (during a limited term) to exclude others from manufacturing, selling, offering for sale or using the patented invention without the patentee's permission.

Patent offices deal with prior art searches in the context of the patent granting procedure. A patent search is frequently carried out by patent offices or patent applicants in order to identify relevant prior art. Certain patent offices may also rely on the patent search results of other patent offices or cooperate with other patent offices in order to identify relevant prior art. Prior art may also be submitted by the public for consideration in examination or in opposition or invalidity proceedings. Relevant prior art identified by patent offices or patent applicants are often cited by patent applicants in patent applications and by patent offices in patent search reports.

Prior art may comprise information that is disclosed to the public in written form, oral form, or by use. Sources of disclosure in written form may include published patents or patent applications or scientific and technical books and journals. Unpublished patent applications may also be considered prior art under certain circumstances, for example where an unpublished patent application was filed at the same patent office before the effective filing date of the patent application in question.

To anticipate the subject-matter of a patent claim, prior art is generally expected to provide a description sufficient to inform an average worker in the field (or the person skilled in the art) of some subject matter falling within the scope of the claim. Prior art must be available in some way to the public, and in many countries, the information needs to be recorded in a fixed form somehow.

Traditional knowledge, such as traditional medicine, may be considered prior art. Information covered by non-disclosure agreements or similar may not be considered to have been disclosed to the public and thus not prior art.

If an invention has been described in the prior art or would have been obvious from what has been described in the prior art, a patent on that invention is not valid.

Information kept secret, for instance, as a trade secret, is not usually prior art, provided that employees and others with access to the information are under a non-disclosure obligation. With such an obligation, the information is typically not regarded as prior art. Therefore, a patent may be granted on an invention, even though someone else already knew of the invention. A person who used an invention in secret may in some jurisdictions be able to claim "prior user rights" and thereby gain the right to continue using the invention. As a special exception, earlier-filed and unpublished patent applications do qualify as prior art as of their filing date in certain circumstances.