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Ring galaxies are dramatic examples of galaxy transformation caused by a remarkably simple interaction. Observations and numerical models (Lynds & Toomre 1976) argue persuasively that these objects are formed by the near central passage of a companion through a spiral along the rotation axis. The brief additional gravitational force induces epicyclic motions throughout the disk, which act to form radially propagating orbit-crowded rings of gas and stars. The concentration of the ISM into the expanding ring (at the disk's expense) is nearly total and can last for approx 400 Myrs. It is this radical rearrangement of the spiral's ISM that is responsible for their interesting star forming properties.