The Cathedral and the Bazaar: Musings on Linux and Open Source by an Accidental Revolutionary (abbreviated CatB) is an essay, and later a book, by Eric S. Raymond on software engineering methods, based on his observations of the Linux kernel development process and his experiences managing an open source project, fetchmail.

The illustration on the cover of the book is a 1913 painting by Lyubov Popova titled Composition with Figures and belongs to the collection of the State Tretyakov Gallery.

In contrast, Raymond claims that an inordinate amount of time and energy must be spent hunting for bugs in the Cathedral model, since the working version of the code is available only to a few developers.

Raymond points to 19 "lessons" learned from various software development efforts, each describing attributes associated with good practice in open source software development:[3] In 1998, the essay helped the final push for Netscape Communications Corporation to release the source code for Netscape Communicator and start the Mozilla project; it was cited by Frank Hecker and other employees as an outside independent validation of his arguments.

[9] In 1999 Nikolai Bezroukov published two critical essays on Eric Raymond's views of open source software, the second one called "A second look at The Cathedral and the Bazaar".