Abstract. Recent progress in understanding four processes that play a large role in the evolution of clusters of galaxies is reviewed. These are dynamical friction, mergers, collisional tidal stripping and the cluster mean field tide. Recent estimates for the growth rate of the cD galaxy and its frequency of appearance are discussed. In spherical relaxed clusters the theoretical and observational results for the accretion rate of a central massive galaxy seem to be quite consistent. It appears that a major part of the cD formation must occur in subclusters. Recent work on the formation of clusters containing galaxies and dark matter suggests that considerable mass segregation occurs in small subclusters (provided clusters form in a bottom up manner). This appears to be a result of dynamical friction. It may imply that visible clusters are embedded in large dark matter halos and that cluster M/L's have been underestimated.

INTRODUCTION

Clusters of galaxies represent a fascinating, if formidable, challenge for the theorist. Although relatively young in terms of their crossing times (Tcr = R/v), the galaxies are sufficiently large and massive that they interact with each other and the intracluster medium on a timescale comparable to their ages. A sensible way to organize a report on this complicated subject is, by analogy with stellar structure, to report first on the detailed processes which may occur (the ‘hydrodyamics’ and atomic ‘physics’ of the problem) and then to examine the effects of these processes (the analog of ‘evolution’) on the galaxies and the cluster.