In this chapter, we reflect on the aim and objectives of the textbook and address known gaps in our theory coverage. We reinforce the importance of theory in health informatics and review the varying disciplinary origins of the theories considered in the book. We discuss the question of what makes a good theory and how to know which one is relevant for a given study. We recognize the limitations of the body of theory that we have presented and suggest what might be regarded as “native” theory that is original to health informatics. Finally, we propose topics to form a research agenda for theory in health informatics.