Hybrid Logic and its Proof Theory This book is the first book length treatment of hybrid logic and its proof theory. Hybrid logic is an extension of ordinary modal logic which allows explicit reference to individual points in a model. The extra expressive power is useful for many applications for example when reasoning about time one often wants to formulate a series of statements about what happens at specific times. There is little consensus about proof theory for ordinary modal logic. Many modal logical proof systems lack important properties and the relationships between proof systems for different modal logics are often unclear. The present book demonstrates that hybrid logical proof theory remedies this lack of uniformity in ordinary modal logical proof systems. It considers a spectrum of different versions of hybrid logic (propositional first order international first order and intuitionist) and of different types of proof systems for hybrid logic (natural deduction Gentzen tableaux and axiom systems). All these systems can be motivated independently but the fact that the systems can be given in a uniform way shows that hybrid logic and hybrid logical proof theory is a natural enterprise.