My main goal for this paper is to introduce a theory of decision-making – a theory that provides a way of explaining how and why people make the decisions they do, in the middle of complex activities such as teaching. My current research has evolved from my earlier problem solving work on problem solving (Schoenfeld, 1985), so to set the stage for this discussion I will briefly describe that work – what it showed, and the questions it did not answer. That will allow me to describe what a complete theory should be able to accomplish. I then turn to the main body of this paper, three studies of teaching. In those examples I show how, under certain circumstances, it is possible to model the act of teaching, to the point where one can provide a grounded explanation of every decision that a teacher makes during an extended episode of teaching. Following that, I give some other examples to show that the theory is general, and I make a few concluding comments.