Optimal team performance in healthcare is vital to avoid error. Assessing teams improves their performance by providing accurate feedback to team members. To construct an assessment tool that is both valid and reliable, it is necessary to understand what should be measured, when it should be measured and the context of the team performance. Current assessment methods include observation of behavioural markers, self-assessment by team members, event-based coding, and narrative field notes. Future assessment tools should incorporate best practice elements such as having a theoretical or empirical basis for teamwork competencies, measuring process rather than outcomes, analysing rather than describing performance, capturing non-observable cognitions, distinguishing individual from collective team behaviours, using trained raters, and linking assessment to learning objectives.