This paper reports on case study research in the UK analysing the participation of early years staff in interprofessional practice to provide effective care and education for children, primarily those with special educational needs. Even though case study staff in different "outstanding" settings had equivalent qualifications and similar years of experience, the ease with which they were able to marshal and deliver effective provision for children requiring input from other services varied depending on a complex set of factors. Social Practice Theory (Holland and Lave 2009) and the concepts of personal action potency in trajectories of participation (Dreier 2002, 2008) shed light on how and why this was the case. The findings indicate that structural arrangements, interpersonal relationships, history and contentions influenced the practitioners' participation in interprofessional practice, but were not entirely deterministic. Contributing to theoretical development, the paper argues that the potential for interprofessional practice is a shared rather than individual capacity in settings. There is scope for enhancing this capacity through attention to the features identified in the analysis. (Contains 2 tables and 1 note.)