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.