  Most ultra-compact dwarf galaxies (UCDs) and very massive globular clusters
reside in nearby galaxy clusters or around nearby giant galaxies. Due to their
distance (>Mpc) and compactness (r_eff<100pc) they are barely resolved, and
thus it is difficult to obtain their internal properties. Here I present our
most recent attempts to constrain the mass function, stellar content and
dynamical state of UCDs in the Fornax cluster. Thanks to radial velocity
membership assignment of ~950 globular clusters (GCs) and UCDs in the core of
Fornax, the shape of their mass function is well constrained. It is consistent
with the 'standard' Gaussian mass function of GCs. Our recent simulations on
the disruption process of nucleated dwarf galaxies in cluster environments
showed that ~40% of the most massive UCDs should originate from nuclear star
clusters. Some Fornax UCDs actually show evidence for this scenario, as
revealed by extended low surface brightness disks around them and onsets of
tidal tails. Multi-band UV to optical imaging as well as low to medium
resolution spectroscopy revealed that there exist UCDs with youngish ages,
(sub-)solar [alpha/Fe] abundances, and probably He-enriched populations.
