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 400 Myrs. It is this radical
rearrangement of the spiral's ISM that is responsible for their
interesting star forming properties.