  N-body simulations of disc galaxies that display recurrent transient spiral
patterns are comparatively easy to construct, but are harder to understand. In
this paper, I summarise the evidence from such experiments that the spiral
patterns result from a recurrent spiral instability cycle. Each wave starts as
rapidly growing, small-amplitude instability caused by a deficiency of
particles at a particular angular momentum. The resulting large-amplitude wave
creates, through resonant scattering, the conditions needed to precipitate a
new instability.
