In this paper we develop the theoretical foundations to exploit symmetries in modal logics. We generalize the notion of symmetries of propositional formulas in conjunctive normal form to modal formulas using the framework provided by coinductive modal models introduced in [5]. Hence, the results apply to a wide class of modal logics including, for example, hybrid logics. We present two graph constructions that enable the reduction of symmetry detection in modal formulas to the graph automorphism detection problem, and we evaluate the graph constructions on modal benchmarks. §1. Symmetries in Automated Theorem Proving. Symmetry is a familiar notion. Intuitively, we say that an object is symmetric if "under any kind of transformation at least one property of the object is left invariant" [25]. Symmetry has many uses. Not only can we study the symmetric properties of an object (geometric, mathematical, etc.) to understand its behavior, but we can also derive specific consequences regarding the object under study based on its symmetry properties, i.e., using a "symmetry-based argument" [33]. In automated reasoning, many problem classes, in particular those arising from real world applications, present symmetries, and their presence is usually a source of additional complexity since we might end up looking for solutions in symmetrical subspaces of the problem's search space. Ideally, if we can recognize that such symmetries exist, we might use them to direct a search algorithm to look for solutions only in nonsymmetric parts of the search space, thus reducing the overall difficulty [42].In this context, a symmetry can be defined as a permutation of the variables (or literals) of the input formula that preserves its structure and its solutions. Symmetries can be classified into semantic or syntactic [14]. Semantic symmetries are permutations of the formula that preserves its set of the models (or solutions) and, therefore, can be regarded as properties of the underlying Boolean function, independent of the particular syntactic representation. For example, consider the Boolean function f(a, b, x, y) = axy + bxy, where a, b, x, y take values over {0, 1} and sum and product are binary. It is straightforward to verify that the permutation that interchanges the parameters a and b leaves the function unchanged, maintaining its set of solutions. Syntactic symmetries, on the other hand, correspond to the