This book introduces readers to multicriteria decision aiding (MCDA) interventions used in complex situations. In each chapter, expert analysts propose a piece of the puzzle, while the final, complete puzzle offers an interesting reflection of the main challenges and difficulties associated with decision aiding interventions, as well as the different tools adopted in response. Particular attention is given to the approaches used to identify and overcome specific difficulties. Designed for analysts familiar with several multicriteria methods but seeking detailed information and experience, this book helps to elaborate and validate MC models in real-life decision aiding interventions. In addition, it helps novice analysts appreciate the difficulty of decision aiding and use the available methods to reduce or control them with the help of socio-technical approaches.