The better the team mental model, the better the teamwork. An important aspect of what determines a good team model is the extent to which the model is shared by the team members. This paper presents suggestions for measuring the extent to which teams have a shared mental model and describes how these measures are related to team performance. The most promising measures of sharedness proposed so far rely on using a compositional approach for team modeling and on a situation-sensitive relevance relation that indicates to what extent components contribute to team performance. A case study illustrates the approach and initial results on measuring performance when teams use different levels of sharedness.