The problem I've been working on for more than 30 years is the challenge of explaining, in theoretical terms, how and why people make the choices they make as they try to solve certain problems-mathematical problems, teaching, building a theory of problem solving, or any other goal-oriented activity. In this talk I'll try to make the case that such behavior can be explained on the basis of individuals' knowledge, goals, beliefs and orientations, and a particular form of decision-making. With luck, the talk will summarize the contents of my new book How We Think, a work in progress as I write this abstract. The main examples will come from detailed studies of teaching.