You might want to create custom components for a number of reasons. For example, you might simply want to add some functionality to an existing component, or you might want to build a reusable component, like a search box or the display of an item in a data grid. You might want to write a completely new kind of component that isn't available in the Flex framework.

If your component is composed mostly of existing components, it is convenient to define it using MXML. However, if it is a completely new kind of component, you should define it as an ActionScript component. For more information, see Creating an ActionScript class.