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 $\Delta$, a $\Delta$-clique is a set of nodes and a time interval such that all pairs of nodes in this set interact at least every $\Delta$ 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.