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 Msol range, between the firmly established stellar-mass black holes and >10^6 Msol 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.