The inventorship question (i.e., "Who is the/an inventor of the claimed invention?") arises in any application filed under the Paris Convention because that convention requires that "the inventor shall have the right to be mentioned as such. . . " (PCT Article 4ter). The answer to the inventorship question becomes critical, however, only when a decision is made to file a patent application in a country that grants patents only to the "first to invent," such as the United States or the Philippines. In such jurisdictions, a patent application must be filed in the name of the actual inventor(s). The inventorship question can arise both before and after the patent application is filed, if the claimed invention changes during prosecution of the application.
The answer to the question is not critical in the filing of international (PCT) patent applications (unless a subsequent national filing in the U.S. is ever desired) or in the filing of patent applications in the European Patent Office (EPO) or Japanese Patent Office (JPO). In the EPO and in the JPO, patents are granted to the applicant(s) who is/are the "first to file."
If more than one person may be involved in conceiving an invention, sole inventorship versus joint inventorship questions can arise. Inventorship questions should be resolved before the patent application is filed because subsequent changes in inventorship can be time consuming and costly. In the U.S., intentionally omitting or adding an inventor can be grounds for invalidating an issued patent.