HST is very well tailored for observations of extragalactic star clusters. One obvious reason is HST's high spatial resolution, but equally important is the wavelength range offered by the instruments on board HST, in particular the blue and near-UV coverage which is essential for age-dating young clusters. HST observations have helped establish the ubiquity of young massive clusters (YMCs) in a wide variety of star-forming environments, from dwarf galaxies and spiral disks to nuclear starbursts and mergers. These YMCs have masses and sizes similar to those of old globular clusters (GCs), and the two may be closely related. A large fraction of all stars seem to be born in clusters, but most clusters disrupt rapidly and the stars disperse to become part of the field population. In most cases studied to date the luminosity functions of young cluster systems are well fit by power-laws dN(L)/dL ~ L^-2, and the luminosity of the brightest cluster can (with few exceptions) be predicted from simple sampling statistics. Mass functions have only been constrained in a few cases, but appear to be well approximated by similar power-laws. The absence of any characteristic mass scale for cluster formation suggests that star clusters of all masses form by the same basic process, without any need to invoke special mechanisms for the formation of YMCs. It is possible, however, that special conditions can lead to the formation of a few YMCs in some dwarfs where the mass function is discontinuous. Further studies of mass functions for star clusters of different ages may help test the theoretical prediction that the power-law mass distribution observed in young cluster systems can evolve towards the approximately log-normal distribution seen in old GC systems.

Invited review for Planets to Cosmology: Essential Science in Hubble's Final Years, ed. M. Livio, STScI, May 2004. 25 pages, including 4 figures and 5 tables