Modal logic deals with inferences and arguments involving the notions of possibility and necessity. This introduction to the semantics and syntax of modal junction and quantifier logic explains the foundations of the theory of possible worlds and provides a calculus of natural reasoning for modal logic proof. It also discusses important philosophical applications of modal logic, as well as issues concerning the existence of possible, necessary, and fictional objects. The book lends itself to either self-study or as a textbook for a basic modal logic course in BA or MA philosophy courses. It is aimed primarily at those who already have an elementary knowledge of logic and wish to deepen it in an important area of philosophical logic.