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Let be a compact oriented surface. A homology cobordism of is a cobordism between two copies of , such that both the “top” inclusion and the “bottom” inclusion induce isomorphisms in homology. Homology cobordisms of form a monoid, into which the mapping class group of embeds by the mapping cylinder construction. In this chapter, we survey recent works on the structure of the monoid of homology cobordisms, and we outline their relations with the study of the mapping class group. We are mainly interested in the cases where is empty or connected.