Four competing theories of health-protective behavior are reviewed: the health belief model, the theory of reasoned action, protection motivation theory, and subjective expected utility theory. In spite of their commonalities, these models are seldom tested against one another. The review points out the similarities and differences among these theories and the data and analyses needed to compare them. In addition to describing the content of the models, their conceptualization of key variables, and the combinatorial rules used to make predictions, some general problems in theory development and testing for health behaviors are examined. The article's goal is to help investigators design studies that will clarify the strengths and weaknesses of these models, leading toward a better understanding of health behavior.