  Intermediate mass black holes (IMBHs) are an (as yet) elusive class of black
holes that are expected to lie in the $10^2-10^5\,M_{\odot}$ range, between the
firmly established stellar-mass black holes and ${\gtrsim}10^6\,M_{\odot}$
supermassive black holes. Predicted by a variety of theoretical models, IMBHs
are the potential seeds of supermassive black holes and are expected to power
some of the brightest extra-nuclear X-ray sources. This brief review is the
result of a presentation and subsequent discussion of IMBHs that took place
during the 12th International Frascati workshop on "Multifrequency Behaviour of
High Energy Cosmic Sources". The manuscript aims to provide a concise and
up-to-date review of the different evolutionary scenarios for the creation of
IMBHs. Throughout the text I emphasize the importance of the identification and
classification of IMBHSs in our effort to understand the formation of
supermassive black holes and their co-evolution with their host galaxies.
