A layout engine, or rendering engine, is software that takes web content (such as HTML, XML, image files, etc.) and formatting information (such as CSS, XSL, etc.) and displays the formatted content on the screen. It "paints" on the content area of a window, which is displayed on a monitor or a printer. A layout engine is typically used for web browsers, email clients, or other applications that require the displaying (and editing) of web contents.

The term "layout engine" only reached popular usage when these became easily separable from the browser. For example Trident, the layout engine from Internet Explorer, is used by many applications on the Microsoft Windows platform to render HTML, as in the mini-browser in Winamp. The Mozilla project's layout engine, Gecko, is commonly referred to only as a layout engine, as unlike with Trident or Opera's Presto, the main application (the Mozilla application suite) is less used than other browsers based on the engine such as Mozilla Firefox.

• List of layout engines
• Comparison of layout engines
  • Comparison of layout engines (HTML)
  • Comparison of layout engines (XML)
  • Comparison of layout engines (XHTML)
  • Comparison of layout engines (graphics)
  • Comparison of layout engines (CSS)
  • Comparison of layout engines (DOM)