  Nuclear star clusters are among the densest stellar systems known and are
common in both early- and late-type galaxies. They exhibit scaling relations
with their host galaxy which may be related to those of supermassive black
holes. These may therefore help us to unravel the complex physical processes
occurring at the centres of galaxies. The properties of nuclear stellar systems
suggest that their formation requires both dissipational and dissipationless
processes. They have stellar populations of different ages, from stars as old
as their host galaxy to young stars formed in the last 100 Myr. Therefore star
formation must be happening either directly in the nuclear star cluster or in
its vicinity. The secular processes that fuel the formation of pseudobulges
very likely also contributes to nuclear star cluster growth.
