Modal logic originated in philosophy as the logic of necessity and possibility. Now it has reached a high level of mathematical sophistication and has many applications in a variety of disciplines, ranging from artificial intelligence, computer science and mathematics via natural language syntax and semantics to philosophy. It is the main source of formal languages aimed at analyzing complex notions such as common knowlwdge and formal provability. This volume presents a broad, contemporary view of the field, with contributions covering the foundations of modal logic as well as its many application areas.