  Black holes with hundreds to thousands of solar masses are more massive than
can be formed from a single star in the current universe, yet the best
candidates for these objects are not located in gas-rich environments where
gradual accretion could build up the mass. Three main formation scenarios have
been suggested in the literature: that intermediate-mass black holes are the
remnants of the first, metal-poor, stars; that they result from direct
collisions in young stellar clusters; or that they are produced by gradual
interactions and mergers of compact objects in old dense clusters. We discuss
each of these in turn and speculate on future observations that may help
sharpen our understanding of the formation of intermediate-mass black holes.
