This document summarizes the history of Yahoo's open source software (OSS) efforts and the development of YUI. It describes how in 1996 Yahoo began using open source software internally and how by 2002 it was releasing some of its own software as open source. In 2006, Yahoo released the first version of YUI, a JavaScript library for building richly interactive web applications, and continued expanding it through numerous releases. Over the following years Yahoo grew YUI significantly, established an open governance model, and helped foster a community of external contributors to YUI and related projects.