Shirley Gregor's research essay examines the structural nature of theory in Information Systems (IS). Despite the importance of theory, questions about its form and structure have been neglected compared to epistemological questions. The essay addresses issues of causality, explanation, prediction, and generalization that underlie understanding theory. A taxonomy is proposed that classifies IS theories based on how they address four central goals: analysis, explanation, prediction, and prescription. Five types of theory are distinguished: (1) theory for analyzing, (2) theory for explaining, (3) theory for predicting, (4) theory for explaining and predicting, and (5) theory for design and action. Examples illustrate each theory type. The taxonomy is demonstrated by classifying a sample of journal articles. The paper shows that multiple views of theory exist and exposes the assumptions underlying different viewpoints. It also suggests that the type of theory being developed can influence the choice of an epistemological approach. The essay argues for the development of integrated bodies of theory that encompass all theory types. Keywords include theory, theory taxonomy, theory structure, information systems discipline, philosophy of science, philosophy of social sciences, interpretivist theory, design theory, design science, explanation, prediction, causality, generalization. The essay discusses the structural nature of theory in IS, the need for generalization, the nature of causality, and the core goals of explanation and prediction. It also explores different perspectives on theory, including the interpretivist tradition, the philosophy of science, and the sciences of the artificial. The essay concludes that theory in IS should be seen as an abstract entity that aims to describe, explain, and enhance understanding of the world, and that the structural nature of theory in IS is distinct from other disciplines. The essay proposes a taxonomy for classifying theories in IS based on their goals, and discusses the implications of this taxonomy for research and practice.Shirley Gregor's research essay examines the structural nature of theory in Information Systems (IS). Despite the importance of theory, questions about its form and structure have been neglected compared to epistemological questions. The essay addresses issues of causality, explanation, prediction, and generalization that underlie understanding theory. A taxonomy is proposed that classifies IS theories based on how they address four central goals: analysis, explanation, prediction, and prescription. Five types of theory are distinguished: (1) theory for analyzing, (2) theory for explaining, (3) theory for predicting, (4) theory for explaining and predicting, and (5) theory for design and action. Examples illustrate each theory type. The taxonomy is demonstrated by classifying a sample of journal articles. The paper shows that multiple views of theory exist and exposes the assumptions underlying different viewpoints. It also suggests that the type of theory being developed can influence the choice of an epistemological approach. The essay argues for the development of integrated bodies of theory that encompass all theory types. Keywords include theory, theory taxonomy, theory structure, information systems discipline, philosophy of science, philosophy of social sciences, interpretivist theory, design theory, design science, explanation, prediction, causality, generalization. The essay discusses the structural nature of theory in IS, the need for generalization, the nature of causality, and the core goals of explanation and prediction. It also explores different perspectives on theory, including the interpretivist tradition, the philosophy of science, and the sciences of the artificial. The essay concludes that theory in IS should be seen as an abstract entity that aims to describe, explain, and enhance understanding of the world, and that the structural nature of theory in IS is distinct from other disciplines. The essay proposes a taxonomy for classifying theories in IS based on their goals, and discusses the implications of this taxonomy for research and practice.