Object-Z is an object-oriented extension of the formal specification language Z. It adds to Z notions of classes and objects, and inheritance and polymorphism. By extending Z's semantic basis, it enables the specification of systems as collections of independent objects in which self and mutual referencing are possible. This work presents a description of Object-Z including discussions of semantic issues, definitions of all language constructs, type rules and other rules of usage, specification guidelines, and a full concrete syntax.