A logical theory should provide a general criterion of validity. "Relevant Logic" sets out to establish such a criterion, and to describe the philosophical basis and the formal theory of logical argument. The notion of relevance required for this theory is obtained by an analysis of the grounds for asserting a formula in a proof. Stephen Read essays a deduction of proof-theory embracing classical, relevant and modal logics and consistent with the appropriate semantics. "Relevant Logic" also contains a philosophical analysis of the notion of meaning as it relates to logical theory. The work is aimed at specialists in philosophical logic, formal logic, cognitive science, semantics and computer science.