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.

This volume contains a number of revised papers that were selected from - pers presented at the last ModelAge workshop held in Certosa di Pertignano (Italy) in 1997, organised by the Institute of Psychology of the Italian CNR (IP-CNR), Division of Arti cial Intelligence, Cognitive Modeling and Inter- tion. The organisation chair was held by Amedeo Cesta. The workshop, and indeed the ModelAge project as a whole, aimed to bring together a number of researchers stemming from di erent disciplines to discuss formal models of agency from di erent perspectives. These disciplines included arti cial intel- gence, software engineering, applied logic, databases, and organisation theory. The e ld of intelligent agents has become an important research area within these disciplines, and in the workshop as in the present volume the concept of agency is thus considered from a multi-disciplinaryperspective. In the introductory chapter of this volume more can be found on the area of intelligent agents as well as on the topic of formal models of these. We f- thermore provide some key references, so that the reader can better appreciate the position of the present volume within the literature on agent technology.