In patent law, an inventor is the person, or persons in United States patent law, who contribute to the claims of a patentable invention. In some patent law frameworks, however, such as in the European Patent Convention (EPC) and its case law, no explicit, accurate definition of who exactly is an inventor is provided. The definition may slightly vary from one European country to another. Inventorship is generally not considered to be a patentability criterion under European patent law.

Under older U.S. case law, an inventor is the one with "intellectual domination" over the inventive process and not merely one who assists in its reduction to practice. In the contemporary U.S. patent law inventor is defined as the person, who "conceived a claim". The persons who only made prototypes or suggested improvements not claimed in a patent are not inventors.

As the patent application moves through an examination at the USPTO, the original application is usually split into two or more divisional patent applications, which may have different inventors, because of different claims in the applications, and therefore different people, who conceived the claims. Therefore, it is important to write down during the filing of the very first application for each claim, who conceived it.

"Joint inventors", or "co-inventors", exist when a patentable invention is the result of inventive work of more than one inventor. Joint inventors may exist even where one inventor contributed a majority of the work.

Under the European Patent Convention (EPC), identifying the inventor of a given invention is theoretically very important since "[t]he right to a European patent (...) belong[s] to the inventor or his successor in title", according to the first-to-file principle. In practice however, the European Patent Office (EPO) never investigates whether the proposed inventor is indeed the true inventor. Indeed, "[f]or the purposes of proceedings before the [EPO], the applicant shall be deemed to be entitled to exercise the right to the European patent".

Court actions relating to the entitlement to the grant of a European patent must be brought before the national court which has the jurisdiction. The jurisdiction is determined in conformance with the "Protocol on Jurisdiction and the Recognition of Decisions in respect of the Right to the Grant of a European Patent" or, in short, the "Protocol on Recognition". Once a final decision is issued by a national court adjudging that the applicant is not entitled to the grant of a European patent, the procedure according to Article 61 EPC is applicable.

In contrast with U.S. patent law, the applicant for a European patent need not be the inventor. The right to the European patent may validly be transferred before the filing of the application, e.g. by contract, by inheritance, or as a consequence of the "employee's rights" as determined by the applicable national law. However, the inventor has the right to be mentioned as such before the EPO. The EPO does not verify the accuracy of the designation of the inventor. The inventor may waive its right to be mentioned as such in the European patent application and European patent specification.

Inventorship is traditionally not classified as a patentability criterion under European patent law, in contrast with U.S. patent law. However, inventorship can be relevant to patentability in Europe, although in only a limited way. Where a disclosure is made within 6 months preceding the filing date of a European patent application, the disclosure is not taken into consideration if it was due to, or in consequence of, an evident abuse in relation to the applicant or his legal predecessor. Thus the identity of the inventor (who is often the applicant, or their legal predecessor) can be of vital importance.