ActionScript is the programming language that the user can use along with MXML to create sophisticated Flex applications. While MXML is an important part of a Flex application, it is mostly used for creating the user interface, and it can go only so far in creating a complete application. For data models and sophisticated client-side business logic, the user will need to use ActionScript as well. Flex applications require ActionScript 3.0, which represents a significant maturation from earlier versions of the language. ActionScript 3.0 is compliant with the ECMA- 262 specification and leverages parts of the pending ECMAScript Edition 4 specification.