This chapter introduces the idea of theories in health informatics, defines what we mean by theory and distinguishes theories from models, frameworks and predictive principles. After explaining why theories and predictive principles are needed to help us professionalize our discipline, the chapter offers five criteria for a successful predictive principle, discusses how to evaluate predictive principles and theories and links this with the emerging field of evidence-based health informatics. The chapter concludes with three actions needed to move the discipline of theory-based health informatics forward.