User Interface Library (YUI) is a discontinued open-source JavaScript library for building richly interactive web applications using techniques such as Ajax, DHTML, and DOM scripting.

Among the enhancements are a CSS selector driven engine, like jQuery, for retrieving DOM elements, a greater emphasis on granularity of modules, a smaller seed file that loads other modules when necessary, and a variety of syntactic changes intended to make writing code faster and easier.

co-founder Jerry Yang; its principal architects have been Sha, Adam Moore, and Matt Sweeney.

Future development will be limited to maintenance releases addressing issues that are "absolutely critical to Yahoo properties.

The YUI Core is a light (31KB minified) set of tools for event management and DOM manipulation.