Mathematics teachers’ professional learning has frequently been treated as either predominantly participationist or acquisitionist – as social learning in becoming part of a community of practice or cognitive learning in the acquisition of knowledge and beliefs. Our experience within mathematics education suggests that professional learning is neither purely acquisitionist nor participatory and that a theoretically integrated treatment of both, together with affective processes of learning, helps shed more light on the complexities of professional learning. In this theoretical paper we firstly show how Social Cognitive Theory and its component theory of self-efficacy attend to this integration; and, secondly, illustrate this integration with an example from our research into preservice mathematics teachers’ training.