This book is the result of a three day workshop on "Evaluation in theory and practice in spatial planning" held in Ramsey Hall, University College London, in September 1996. Some 30 people from 8 different countries attended and 20 papers were presented. The majority of them now form the basis for this book. This occasion was the third on the topic, the two preceding having taken place in Umea in June 1992 and in Bari in 1994. Following these three meetings, we can now say that this small, industrious, international family really enjoy meeting up from time to time at each others places, in the presence of older members and new children, each one presenting his/her own recent experiences. It particularly enjoys exchanging views and arguing about the current state and the future of evaluation in spatial planning (all families have their vices ... ). It is also pleasing to see these experiences and discussions resulting in a book for those who could not attend and for the broader clan inthe field. Not long time ago, but ages in the accelerated academic time scale, evaluation in planning established its own role and distinct features as an instrument for helping the decision-making process. Now this role and these features are exposed to major challenges. First, the evolution of planning theory has lead to the conception of new planning paradigms, based on theories of complexity and communicative rationality.