  This chapter reviews theoretical work on the stellar dynamics of galaxy
disks. All the known collective global instabilities are identified, and their
mechanisms described in terms of local wave mechanics. A detailed discussion of
warps and other bending waves is also given. The structure of bars in galaxies,
and their effect on galaxy evolution, is now reasonably well understood, but
there is still no convincing explanation for their origin and frequency. Spiral
patterns have long presented a special challenge, and ideas and recent
developments are reviewed. Other topics include scattering of disk stars and
the survival of thin disks.
