This modern, advanced textbook on modal logic is intended both for novices and for more experienced readers. The two distinct tracks through the work are clearly signposted at the start of each chapter. The book assumes a prior acquaintance with first-order logic and its semantics and a familiarity with the basic mathematical notions of set theory. The authors focus on the use of modal languages as tools to analyze the properties of relational structures, including algorithmic and algebraic aspects. Applications to issues in logic and computer science such as completeness, computability and complexity are also considered. The work is a must for anyone interested in modal logic.