Modal Logic can be characterized as the logic of necessity and possibility, of 'must be' and 'may be'. A Short Introduction to Modal Logic presents both semantic and syntactic features of the subject and illustrates them by detailed analyses of the three best-known modal systems S5, S4 and T. The book concentrates on the logical aspects of the subject and provides philosophical motivations to show the point of the formal work. The coverage is self-contained, including a summary of the necessary aspects of classical logic which it presupposes. A set of exercises is included in the final chapter.