Editor’s note: This article is part of a series in Medical Education entitled ‘Dialogue’. Each publication in the series will be a transcription of an e-mail discussion about a current issue in the field held by two scholars who have approached the issue from different perspectives. For further details, see the editorial published in Med Educ 2012;46(9):826–7. In this issue, Douglas P Larsen, Director for Medical Student Education for the Division of Paediatric Neurology, Washington University in St Louis, and Tim Dornan, Professor of Medical Education, Faculty of Health, Medicine and Life Sciences, Maastricht University, discuss the roles that testing, broadly defined, and social interaction play in the acquisition and retention of medical knowledge.