The defining feature of relevant logic is that it forces the premises of an argument to be really used and thus become "relevant" in deriving its conclusion. This book introduces the reader to relevant logic and provides it with a philosophical interpretation. The logic is analyzed in the context of possible world semantics and situation semantics, which are then applied to provide an understanding of the various logical particles (especially implication and negation) and natural language conditionals. The book concludes by examining various applications of relevant logic.