Doctors need to identify and understand the professional behaviours of both themselves and others. In order for students to think critically about these issues we encouraged them to use the tenets of the General Medical Council's Duties of a Doctor as a framework in which to reflect on the actions of healthcare professionals at work. Although the critical incident technique is a well-known process for encouraging reflection, little is known about its usefulness for assessment purposes in this setting. We aimed to discover the validity, feasibility and educational impact of the critical incident as an assessment method for first year students undertaking guided reflection in the context of their first exposure to multi-professional health and social care experiences. First year medical students submitted two critical incidents they had observed during multi-professional health and social care attachments and an evaluation of their experiences. Students engaged in the reflective cycle on the professional behaviours of others providing evidence of a varied range of situations. With adequate preparation, junior students are able to reflect on social and healthcare experiences using the Duties of a Doctor as a framework. Critical incidents are a valid and feasible method for assessing students' reflections on professionalism, with good educational impact.