This text is designed to provide a practical introduction to formal specification, explaining the use of mathematically-based techniques for specifying software systems in the formal specification language, Z. The text concentrates on the process of developing a specification and the clear distinction made between the process of constructing a mathematical model and the translation of that model into a formal notation. The process of developing the specification is explained through the exploration of alternative mathematical models and the book emphasizes both model building and the use of a formal notation, in this case Z. Divided into four parts, it begins by introducing the need for formal specification and discusses the general issue of modelling. Part two presents a selection of the basic mathematical tools and applies these to the modelling example. Extended examples are presented in the third part of the book whilst part four places formal specification in the context of formal methods.