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The common language of Flex 2 applications is ActionScript 3.0. You can't use any older versions of ActionScript, nor is it possible to create a functional application without at least some ActionScript 3.0. Additionally, we will soon see that MXML and AS3 are fundamentally related.
ActionScript's third incarnation is a mature language, as well as being substantially different from ActionScript 2. This chapter will rapidly cover all the fundamental aspects of ActionScript 3.0, but is by no means a full course. For more instruction, there is a list of useful further reading at the bottom of this Short Cut.