In this paper we study formal tools for the definition of tableaux systems for modal logics. We begin by some preliminaries about graphs. Then we present graph rewriting rules together with control constructs for defining strategies of application of these rules. We establish fair strategies that are sound, complete and terminating. These results cover almost all basic modal logics such as K, T, S4, KB4, K+Confluence etc., and can be generalized for logics such LTL and PDL. Our framework provides the theoretical basis for our generic tableau theorem prover LoTREC.