Teacher clusters represent a recent experiment in the field of teacher professional development in South Africa. Increasingly, teacher clusters are being used as a substitute for the traditional approaches to professional development in helping teachers reshape their professional knowledge and change their classroom practices. What underlies this renewed confidence in teacher clusters as a vehicle for professional development? In this paper, we use a qualitative case study approach to examine the efficacy of clustering as an approach to teacher development. Using interview and observation data from a cluster of 120 science and mathematics teachers in Mpumalanga, we discuss how structure and function in the cluster interacted to provide the participating science teachers with a rich set of "opportunities to learn". We argue that it is not merely the existence of the structure, namely, the cluster, that provides the opportunity for effective professional development, but that it is the interactions among teachers, together with relationships of trust and identity, that make clusters an attractive vehicle for challenging and (possibly) changing teachers' professional knowledge and practice.