  I review several topics in the structure of supernova remnants. Hydrodynamic
instabilities in young remnants may give rise to the cellular structure that is
sometimes observed, although structure in the ejecta might also play a role.
The presence of ejecta close to the forward shock front of a young remnant can
be the result of ejecta clumps or the dynamical effects of cosmic rays. Slower
moving ejecta clumps can affect the outer shock structure of older remnants
such as Vela. Young remnants typically show a circular structure, but often
have a one-sided asymmetry; the likely reasons are an asymmetric circumstellar
medium, or pulsar velocities in the case of pulsar wind nebulae. In older
remnants, asymmetric pulsar wind nebulae can result from asymmetric reverse
shock flows and/or pulsar velocities.
