A link stream is a sequence of triplets (t, u, v) indicating that an interaction occurred between u and v at time t. We generalize the classical notion of cliques in graphs to such link streams: for a given ∆, a ∆-clique is a set of nodes and a time interval such that all pairs of nodes in this set interact at least every ∆ during this time interval. We propose an algorithm to enumerate all maximal cliques of a link stream, and illustrate its practical relevance on a real-world contact trace.