  In this review, I discuss just three aspects of the stability and evolution
of galactic discs. (1) I first review our understanding of the bar instability
and how it can be controlled. Disc galaxies in which the orbital speed does not
decrease much towards the centre have no difficulty avoiding bars, even when
dark matter makes an insignificant contribution to the inner part of the
rotation curve. (2) I then briefly discuss interactions between disturbances in
the discs of galaxies and the spherical components, which generally exert a
damping effect through dynamical friction. The fact that bars in real galaxies
appear to rotate quite rapidly, seems to require dark matter halos to have
large, low-density cores. (3) In the remainder of the article, I consider the
theory of spiral structure. The new development here is that the distribution
function for stars in the Solar neighbourhood, as measured by HIPPARCOS, is far
less smooth than most theoretical work had previously supposed. The strong
variations in the values of the \DF over small ranges in angular momentum have
the appearance of having been caused by scattering at Lindblad resonances with
spiral patterns. This result, if confirmed when the radial velocity data become
available, supports the picture of spiral patterns as dynamical instabilities
driven by substructure in the \DF. The details of how decaying patterns might
seed conditions for a new instability remain unclear, and deserve fresh
attention.
