Developing instruments to assess teacher competence requires a model of competent performance which can guide both the collection and appraisal of evidence in task situations. Following Kane (1992), the validation of statements about teachers' competence is regarded as the evaluation of interpretive argumentation. Based on contemporary insights into teaching and learning, an interpretive model of competent performance is described which, rather than being prescriptive in nature, offers scope for various forms of responsible professional performance. Consequences of professional performance for students/class/organisation are the basis of the model. Acceptable interventions and underlying decision-making processes as well as the associated parts of a professional knowledge base are derived from the consequences. The consequences of these insights for developing domains of competence and collecting evidence are discussed. (Contains 1 figure.)