This book constitutes the thoroughly refereed postproceedings of the 18th International Workshop on Algebraic Development Techniques, WADT 2006, held in La Roche en Ardenne, Belgium, in June 2006. The 10 revised full papers presented were carefully selected during two rounds of reviewing and improvement. The papers focus on the algebraic approach to the specification and development of systems and address topics such as formal methods for system development, specification languages and methods, systems and techniques for reasoning about specifications, specification development systems, methods and techniques for concurrent, distributed and mobile systems, and algebraic and co-algebraic foundations.