  Various studies have established that the dynamical M/L ratios of
ultra-compact dwarf galaxies (UCDs) tend to be at the limit or beyond the range
explicable by standard stellar populations with canonical IMF. We discuss how
IMF variations may account for these high M/L ratios and how observational
approaches may in the future allow to discriminate between those possibilities.
We also briefly discuss the possibility of dark matter in UCDs.
