This paper describes some recent developments in our understanding of particle acceleration by shocks. It is pointed out that while good agreement now exists as to steady nonlinear modifications to the shock structure, there is. also growing evidence that the mesoscopic scales may not in fact be steady and that siginficant instabilties associated with magnetic field amplification may be a feature of strong collisionless plasma shocks.

The problem of the origin of cosmic rays is considered in an astronomical context and the current observational situation summarised. The evidence for acceleration in supernova remnants is critically examined.