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.
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