A low rumble through the Earth convulses a highway like a fish gasping for air. A child in a distant playground moves gracefully propelling the swing to new heights. A crystal glass shatters with the precision of an operatic singer's voice. The Earth, the child, and the soprano play with oscillations. Not content with the simple vibrations of sound, or a mass on a string, or a screen door swinging to and fro, this active cast of characters forces the systems and produces fascinating results. To understand what is happening, this chapter reviews the simplest vibrating system before exploring the more complex activities of the players.