This research project studied the relationship between teachers' thinking and action, specifically, teachers' implicit theories in the subfield of evaluation. Particular focus was upon the way teachers generate their theories about teaching, the structure and content of such theories, and their influence on decisions and intructional practices. The paper presents some of the elements formulated by a Spanish elementary/middle school mathematics teacher, which make up a theory on the evaluation of teaching. This theory does not reflect a set of scientifically contrasted propositions, but rather a set of experiences confirmed by professional practice. The subjective character of its statements represents the different reflections during the teacher's daily activity in the classroom, thus being subject to a continuous evolution. The structure, which integrates it, is made up of isolated pieces, implicit in the teacher's thinking, which are reached through successive processes of classification and contrast. (JD)