Movie clips were introduced with Flash 4. A movie clip is like a movie within a movie that you can manipulate by using interactive controls (also called actions, and created with ActionScript). Movie clips are crucial for complex animation and especially interactive animation. A movie clip doesn't take place on the main Timeline. Instead, you can go to the movie clip at any time, play it, and then return to where you left off in the Timeline. You can also attach movie clips to buttons.

With Flash 5, you can give a movie clip custom properties. Right-click (Windows) or Control+Click (Mac) the movie clip in the Library and choose Define Clip Parameters. You can then assign specific values to individual instances of movie clips. Select the instance and choose Window, Panels, Clip Parameters. You can add actions to instances of a movie clip; in Flash 4 you were able to place actions only on frames or in buttons.