  In this contribution the discovery and properties of ultra-compact dwarf
galaxies are presented, and their possible origin is discussed. This new type
of galaxy resides in the cores of galaxy clusters. In the fundamental plane
diagram of stellar systems, the luminosity and kinematical and structural
properties of ultra-compact dwarfs locate them inbetween globular clusters and
small compact ellipticals (like M32). This regime would also be occupied by
nuclei of dwarf ellipticals if they were isolated from their galactic envelope
or by the merger product of super star-cluster complexes as they are found in
strongly interacting galaxies. Future investigations of ultra-compact dwarfs in
different environments have to show which of these formation scenarios is to be
favoured.
