Few would argue the notion that teachers’ work is complex. The present article seeks to disentangle and illuminate how complexity is understood within teaching, in particular how and why it is complex, and to discuss it in relation to teacher professionality. Concepts from curriculum theory and research on professionality are used to this end. Complexity in teaching can, for analytical purposes, be divided into aspects such as complexity of meaning, complexity of content, complexity of means and complexity of purposes, which teachers are to deal with. Teacher professionality is conceived as the instantiation of teacher professionalism, which is enacted by a community of professionals. Professionalization and de-professionalization tendencies are apparent in the educational context today. In a sense, the ability to deal well with complexity can be viewed as a basis of teacher professionality. However, the context may or may not contribute to the possibility to act professionally. Hence the question of the relation between professionality and complexity is everything but straightforward.