The Free Software Foundation (FSF) is a non-profit corporation founded by Richard Stallman on 4 October 1985 to support the free software movement, a copyleft-based movement which aims to promote the universal freedom to distribute and modify computer software without restriction. The FSF is incorporated in Massachusetts, USA.

From its founding until the mid-1990s, FSF's funds were mostly used to employ software developers to write free software for the GNU Project. Since the mid-1990s, the FSF's employees and volunteers have mostly worked on legal and structural issues for the free software movement and the free software community.

Being consistent with its goals, only free software is used on all of the FSF's computers.
The FSF holds the copyrights on various essential pieces of the GNU system, such as GNU Compiler Collection. As copyright holder, it has exclusive authority to enforce the GNU General Public License (GPL) when copyright infringement occurs on that software. While other copyright holders of other software systems adopted the GPL as their license, FSF was the only organization to regularly assert its copyright interests on software so licensed until Harald Welte launched gpl-violations.org in 2004.

From 1991 until 2001, GPL enforcement was done informally, usually by Stallman himself, often with assistance with FSF's lawyer, Eben Moglen. Typically, GPL violations during this time were cleared up by short email exchanges between Stallman and the violator.

In late 2001, Bradley M. Kuhn (then Executive Director), with the assistance of Moglen, David Turner, and Peter T. Brown, formalized these efforts into FSF's GPL Compliance Labs. From 2002-2004, high profile GPL enforcement cases, such as those against Linksys and OpenTV, became frequent.

GPL enforcement and educational campaigns on GPL compliance was a major focus of the FSF's efforts during this period.

On December 2008, FSF filed a lawsuit against Cisco for using GPL-licensed components shipped with Linksys. Cisco was notified of the licensing issue in 2003 but Cisco repeatedly disregarded its obligations under the GPL.
In March 2003, SCO filed suit against IBM alleging that IBM's contributions to various free software, including FSF's GNU, violated SCO's rights. While FSF was never a party to the lawsuit, FSF was subpoenaed on November 5, 2003. During 2003 and 2004, FSF put substantial advocacy effort into responding to the lawsuit and quelling its negative impact on the adoption and promotion of free software.