  In the last decade, a new kind of stellar systems has been established that
shows properties in between those of globular clusters (GCs) and early-type
dwarf galaxies. These so-called ultra-compact dwarf galaxies (UCDs) have masses
in the range 10^6 to 10^8 M_sun and half-light radii of 10-100 pc. The most
massive UCDs known to date are predominantly metal-rich and reside in the cores
of nearby galaxy clusters. The question arises whether UCDs are just the most
massive globular clusters in rich globular cluster systems? Although UCDs and
`normal' GCs form a continuous sequence in several parameter spaces, there
seems to be a break in the scaling laws for stellar systems with masses above
~2.5x10^6 M_sun. Unlike GCs, UCDs follow a mass-size relation and their
mass-to-light ratios are about twice as large as those of GCs with comparable
metallicities. In this contribution, I present the properties of the brightest
globular clusters and ultra-compact dwarf galaxies and discuss whether the
observed findings are compatible with a `star-cluster' origin of UCDs or
whether they are more likely related to dark matter dominated dwarf galaxies.
