The Cathedral and the Bazaar is an essay by Eric S. Raymond on open source software engineering methods, based on his experience managing a successful open source project, fetchmail. It was first presented by the author at the Linux Kongress on May 27 1997.

The essay contrasts two different free software development models:

• The Cathedral model, in which access to software source code is restricted to members of the development team. Raymond used GNU Emacs (prior to version 21) and GCC (prior to the EGCS re-merge) as examples: although the source code was available with each release of the software, code developed between releases was restricted to a small and exclusive group of core developers.
• The Bazaar model, in which the code is developed over the Internet in view of the public. Raymond credited Linus Torvalds as the inventor of this process, through the Linux kernel project. He also provided anecdotal accounts of his successful implementation of this model for the fetchmail project.

The essay helped convince most existing open source and free software projects to adopt Bazaar-style open development models, at least partially. It also provided the final push for Netscape to open the source of Netscape Communicator and start the Mozilla project.

The Cathedral is also the typical development model for proprietary software — with the additional restriction in that case that source code is usually not provided even with releases — and a common usage of the phrase "the Cathedral and the Bazaar" is to contrast proprietary with open source. However, the original essay concerns itself only with free software, and does not address proprietary development in any way at all.

The phrase itself has been extended to describe non-software projects. Wikipedia is an example of a Bazaar-style project, while Nupedia or the Encyclopædia Britannica are examples of Cathedral-style project.

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