  The mathematical simplicity of black holes, combined with their links to some
of the most energetic events in the universe, means that black holes are key
objects for fundamental physics and astrophysics. Until recently, it was
generally believed that black holes in nature appear in two broad mass ranges:
stellar-mass (roughly 3-20 solar masses), which are produced by the core
collapse of massive stars, and supermassive (millions to billions of solar
masses), which are found in the centers of galaxies and are produced by a still
uncertain combination of processes. In the last few years, however, evidence
has accumulated for an intermediate-mass class of black holes, with hundreds to
thousands of solar masses. If such objects exist they have important
implications for the dynamics of stellar clusters, the formation of
supermassive black holes, and the production and detection of gravitational
waves. We review the evidence for intermediate-mass black holes and discuss
future observational and theoretical work that will help clarify numerous
outstanding questions about these objects.
