We study the following communication variant of local search. There is some fixed, commonly known graph G. Alice holds f A and Bob holds f B , both are functions that specify a value for each vertex. The goal is to find a local maximum ofOur main result is that finding a local maximum requires polynomial (in the number of vertices) bits of communication. The result holds for the following families of graphs: three dimensional grids, hypercubes, odd graphs, and degree 4 graphs. Moreover, we provide an optimal communication bound of Ω( √ N ) for the hypercube, and for a constant dimensional greed, where N is the number of vertices in the graph.We provide applications of our main result in two domains, exact potential games and combinatorial auctions. First, we show that finding a pure Nash equilibrium in 2-player Naction exact potential games requires polynomial (in N ) communication. We also show that finding a pure Nash equilibrium in n-player 2-action exact potential games requires exponential (in n) communication.The second domain that we consider is combinatorial auctions, in which we prove that finding a local maximum in combinatorial auctions requires exponential (in the number of items) communication even when the valuations are submodular.Each one of the results demonstrates an exponential separation between the non-deterministic communication complexity and the randomized communication complexity of a total search problem. {1, ..., W }, and their goal is to find aDetermining the communication complexity of SumLS on certain families of graphs is easy. For example, a simple reduction from disjointness shows that the communication complexity of SumLS on the clique with n vertices is Ω(n). Our main theorem proves optimal lower bounds for several important families of graphs, all have small degree. The technical challenge is that the nondeterministic communication complexity of the problem on small degree graphs is clearly low: to verify that v * is a local optimum, Alice and Bob need only communicate the values f (u) and g(u) for the small number of v * 's neighbours in the graph (note that the degree of all graphs that we consider is indeed small: log N or even constant). There are only a few results in the communication complexity literature that manage to prove good lower bounds for total problems where verification is easy, most notably for Karchmer-Wigderson games [Karchmer and Wigderson, 1990, Karchmer et al., 1995, Raz and McKenzie, 1997] and for PPAD-like communication problems [Babichenko and Rubinstein, 2016, Göös andRubinstein, 2018].Main Theorem.