Two important new concepts, module specifications and constraints, are introduced in this second volume of a three-volume set on fundamentals of algebraic specification. These concepts are motivated by problems in practical software development and are studied here from a theoretical point of view. Modularization is one of the main structuring principles in software development. Modules and module specifications can be seen as the basic building blocks which are used in modularization of software systems and software system specification. Constraints are introduced to increase the expressive power of algebraic specifications in order to make them more useful for practical applications. The book is suitable both as a textbook for graduate courses in formal specification of data types and software systems and as a reference for researchers and system developers.