Macromedia Flex (previously code-named Royale) extends the Macromedia MX platform by delivering a standards-based programming methodology for building the presentation tier of Rich Internet Applications.

Flex is a presentation server installed on top of a J2EE application server or servlet container, a rich library of user interface components, an XML-based markup language used to declaratively lay out these components, and an object-oriented programming language which handles user interactions with the application. The result is a Rich Internet Application rendered using Flash Player and developed using industry standards and a development paradigm familiar to developers. Flex runs on top of a J2EE Application Server such as Macromedia JRun, IBM Websphere, BEA WebLogic, or Apache Tomcat. In the future, Flex will also run on top of Microsoft .NET server.