This book constitutes the strictly refereed post-workshop proceedings of the 12th International Workshop on Algebraic Development Techniques, WADT '98, held in Tarquinia, Italy, in June 1997.This book presents, besides three invited surveys, 21 carefully revised full papers selected from 40 presentations given at the workshop. The algebraic approach to the specification and development of systems, born as a formal method for abstract data types, encompases today the formal design of integrated hardware and software systems, new specification frameworks and a wide range of applications. This book presents the state-of-the-art in the area of algebraic software development.