The problem of detecting network structures plays a central role in distributed computing. One of the fundamental problems studied in this area is to determine whether for a given graph H, the input network contains a subgraph isomorphic to H or not. We investigate this problem for H being a clique K in the classical distributed CONGEST model, where the communication topology is the same as the topology of the underlying network, and with limited communication bandwidth on the links.Our first and main result is a lower bound, showing that detecting K requires Ω( √ n/b) communication rounds, for every 4 ≤ ≤ √ n, and Ω(n/( b)) rounds for every ≥ √ n, where b is the bandwidth of the communication links. This result is obtained by using a reduction to the set disjointness problem in the framework of two-party communication complexity. We complement our lower bound with a two-party communication protocol for listing all cliques in the input graph, which up to constant factors communicates the same number of bits as our lower bound for K4 detection. This demonstrates that our lower bound cannot be improved using the two-party communication framework. detecting K 4 and K in Section 2, to ensure full generality of presentation, we will make the analysis parametrized by the message size b, in which case we will refer to such model of distributed computation as CONGEST b , the CONGEST model with messages of size b.)Our goal is, for a given network G = (V, E) and ≥ 4, to solve the subgraph detection problem for a clique K , that is, to design an algorithm in the CONGEST model such that (i) if G contains a copy of K , then with probability at least 2 3 at least one node outputs 1, and (ii) if G does not contain any copy of K , then with probability at least 2 3 no node outputs 1. The subgraph detection problem is a local problem: it can be solved efficiently solely on the basis of local information. In particular, in the CONGEST model, the problem of finding K in a graph can be trivially solved in O(n) rounds, or in fact, in O(max u∈V deg G (u)) rounds, where deg G (u) denotes the degree of node u in G. Indeed, if each node sends its entire neighborhood to all its neighbors, then afterwards, each node will be aware of all its neighbors and of their neighbors. Therefore, in particular, each node will be able to detect all cliques it belongs to. Since for each node u, the task of sending its entire neighborhood to all its neighbors can be performed in O(deg G (u)) rounds in the CONGEST model, the total number of rounds for the entire network is O(max u∈V deg G (u)) = O(n) rounds. In view of this simple observation, the main challenge in the clique K detection problem is whether this task can be performed in a sublinear number of rounds. n i=1 X i ∧ Y i . In a seminal work, Kalyanasundaram and Schnitger [13] showed that in any randomized communication protocol, the players must exchange Ω(n) bits to solve the set disjointness problem with constant success probability.Theorem 1 ([13]). The randomized two-party communication comp...