Stallman announced the plan for the GNU operating system in September 1983 on several ARPANET mailing lists and USENET.<ref name="newunix"/>
Stallman started the project on his own and describes: "As an operating system developer, I had the right skills for this job. So even though I could not take success for granted, I realized that I was elected to do the job. I chose to make the system compatible with Unix so that it would be portable, and so that Unix users could easily switch to it."<ref name="gnuproject"/>
In 1985, Stallman published the GNU Manifesto, which outlined his motivation for creating a free operating system called GNU, which would be compatible with Unix.<ref name="The Wikipedia Revolution by Andrew Lih"/> The name GNU is a recursive acronym for "GNU's Not Unix."<ref name="The Wikipedia Revolution by Andrew Lih"/> Soon after, he started a nonprofit corporation called the Free Software Foundation to employ free software programmers and provide a legal infrastructure for the free software movement. Stallman is the nonsalaried president of the FSF, which is a 501(c)(3) nonprofit organization founded in Massachusetts.<ref name="DuBois"/> Stallman popularized the concept of ''copyleft'', a legal mechanism to protect the modification and redistribution rights for free software. It was first implemented in the GNU Emacs General Public License, and in 1989 the first program-independent GNU General Public License (GPL) was released. By then, much of the GNU system had been completed.
Stallman was responsible for contributing many necessary tools, including a text editor (Emacs), compiler (GCC (GNU Compiler Collection)), debugger (GNU Debugger), and a build automator (build automation) (GNU make). The notable omission was a kernel (kernel (computer science)). In 1990, members of the GNU began using Carnegie Mellon's Mach microkernel in a project called GNU Hurd, which has yet to achieve the maturity level required for full POSIX compliance.
In 1991, Linus Torvalds, a Finnish (Finns) student, used the GNU's development tools to produce the free monolithic (monolithic kernel) Linux kernel. The existing programs from the GNU project were readily ported to run on the resultant platform. Most sources use the name ''Linux'' to refer to the general-purpose operating system thus formed, while Stallman and the FSF call it ''GNU/Linux''. This has been a longstanding naming controversy (GNU/Linux naming controversy) in the free software community. Stallman argues that not using GNU in the name of the operating system unfairly disparages the value of the GNU project and harms the sustainability of the free software movement by breaking the link between the software and the free software philosophy of the GNU project.