One of the most important tasks when studying networks is that of identifyingnetwork communities.Fundamentally, communities allow us to discover groups of interacting objects(i.e.,nodes) and the relations between them. For example, in social networks, communities correspond to groups of friends who attended the same school, or who come from the same hometown; in protein interaction networks, communities are functional modules of interacting proteins; in co-authorship networks, communities correspond to scientific disciplines. Identifying network communities allows us to discover functionally related objects , study interactions between modules, infer missing attribute values and predict unobserved connections.