Service Science is the study of co-creative systems. The ever-broadening interest in Service Innovation pursues the effective and efficient design and management of co-creative systems, which can be accomplished only through the application of a science of these systems. Examples of such service include users navigating web sites, people interacting with intelligent mobile applications, customers in a shopping mall, patients interacting with health-care professionals and other sources of advice, students interacting with teachers and learning materials, etc. The trajectories of agents that engage in such co-creative systems are directed by the decisions made by agents. Hence, understanding the performance of service systems and designing better service systems requires understanding how these agents make their decisions and how service systems respond to these decisions. This book defines and explains the essential foundational constructs for graphical and mathematical models of co-creative systems.