Book excerpt: This book examines the theoretical foundations of the processes of planning and design. When people – alone or in groups – want to solve problems or improve their situation, they make plans. Horst Rittel studied this process of making plans and he developed theories – including his notion of "wicked problems" – that are used in many fields today. From product design, architecture and planning – where Rittel’s work was originally developed – to governmental agencies, business schools and software design, Rittel’s ideas are being used. This book collects previously unavailable work of Rittel’s within the framework of a discussion of Rittel’s theories and philosophical influences.