This document provides a history of the development of the open source operating system illumos. It describes how illumos originated from OpenSolaris, which itself originated from SunOS and Solaris. It discusses key innovations in Solaris and the rise of open source development. It outlines challenges around open sourcing Solaris and the community challenges that emerged. It details how illumos was created after Oracle discontinued support for OpenSolaris, leading many developers to continue their work in illumos. It argues illumos has become the repository for many important operating system innovations.