Abstract
I propose to give a review on ultra-compact dwarf galaxies (UCDs), a new class of stellar systems defining the interface between star clusters and dwarf galaxies. UCDs are believed to be either the most massive star clusters in the universe, tidally truncated galaxies, or, both. After a brief overall summary, I will focus in particular on two aspects. 1. The specific frequencies of UCDs - a recently introduced quantity that allows to test whether the luminosity distribution of UCDs follows the bright tail of the globular cluster luminosity function. Can all UCDs be explained as bright star clusters? 2. The elevated dynamical M/L ratios of UCDs. Are they due to highly clustered dark matter, a variation of the IMF, or maybe, due to massive central black holes that would be fossil relicts of UCD progenitor galaxies?
