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 _ { \odot } 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 \sim 2.5 \times 10 ^ { 6 } M _ { \odot } .
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 .