Since the 1970s, interagency training has been considered crucial for developing effective collaborative practice, initially in relation to child protection, and latterly, to safeguarding children's welfare. Charting the history of interagency training in England, this paper aims to demonstrate how, despite limited attention being paid to its evaluation, the role of training has expanded in terms of both content and target audience. This raises questions about the evidence base upon which investment in interagency training is founded. Drawing on the limited literature and experiences of interagency trainers, the authors question whether this investment is an act of faith or reason. They conclude that a belief in the value of training different disciplines together persists, despite little being known about the way in which interagency training improves practice.