Usually the term "patentability" only refers to "substantive" conditions, and does not refer to formal conditions such as the "sufficiency of disclosure", the "unity of invention" or the "best mode requirement".
Judging patentability is one aspect of the official examination of a patent application performed by a patent examiner. Although the grant of a patent creates a presumption that the claimed invention is valid, errors in the granting procedure may occur and previously unconsidered prior art may be brought to light only after the patent was granted, but under higher scrutiny based on the presumption of validity.
Prior to filing a patent application, inventors sometimes obtain a patentability opinion from a patent agent or patent attorney regarding whether an invention satisfies the substantive conditions of patentability.
Many national and regional patent offices provide procedures for reconsidering whether or not a given patent is valid after grant. Under the European Patent Convention, any person can file an opposition provided they act promptly after grant of the patent. In the United States, members of the public can initiate reexamination proceedings. Japan provides similar options as well.
Members of the public can also initiate lawsuits in the courts of various nations to have patents declared invalid.
United Kingdom patents can be reviewed by way of a non-binding opinion issued by the Patent Office, or by formal applications for revocation before the Patent Office or the Court. If the patent survives a revocation action, this is noted for future reference by way of a Certificate of contested validity.
The fact that an invention is patentable does not necessarily mean that that invention does not also infringe another patent. The first patent in a given area may have a broad claim covering the concept of the invention since there is no prior art in that area. Later, a specific implementation of that concept may be invented, which is patentable as it is not disclosed in the earlier patent, but that falls within the claim to the general concept. The later inventor must, therefore, obtain a licence from earlier proprietor to be able to exploit his invention.
Thomas Edison's thin carbon filament light bulb was a patentable improvement over the earlier patented Woodward and Evans thick carbon filament light bulb. Thomas Edison bought the Woodward patent for $US 5,000 before he began his development work so that Woodward would not be able to sue him for patent infringement after Edison became commercially successful.
Under United States patent law, inventorship is also regarded as a patentability criterion. It is a constitutional requirement. Congress' ability to grant patents is authorized only for the inventor. This was confirmed by case law: "Inventorship is indeed relevant to patentability under 35 U.S.C. § 102(f), and patents have in the past been held unenforceable for failure to correctly name inventors in cases where the named inventors acted in bad faith or with deceptive intent."
Details on patentability in the U.S. can be found in the Manual of Patent Examining Procedure or MPEP. This is published by the United States Patent and Trademark Office (USPTO) and is the reference manual used by both patent examiners and patent agents/attorneys. Chapter 2100, in particular, gives a comprehensive overview of the standards for patentability, a discussion of the related case law, and guidance on how to overcome an examiner's rejection of a given set of claims.
In the United States, the patent grant is presumptive, e.g. a patent shall issue unless the patent statutes preclude the grant. In other words, the burden is on the Patent Office to prove why a patent should NOT be granted.
See List of patent legal concepts for articles on various legal aspects of patents, including special types of patents and patent applications.