Free and open-source software (FOSS) licences are licences for computer programs.

FOSS licences are pre-formulated non-exclusive licence agreements for computer programs. They can be used by programmers to license their rights to use their computer programs and make the editable source code of their computer programs available to all interested users.

There are a large number of FOSS licences. The Free Software Foundation maintains a list of all licences that fall under the definition of free software. The Open Source Initiative maintains a list of all licences that meet the definition of open source software. The two lists are almost congruent, which is why the name free and open-source software is often used – albeit inaccurately.

To summarise, free and open source software is often defined as software which can be freely run, studied, distributed and modified by users. This presupposes that FOSS licences grant comprehensive rights to use, and guarantee access to the editable source code.