In this paper we operationally define and measure tacit knowledge at the team-level in the software development domain. Through a series of three empirical studies we developed and validated the team tacit knowledge measure (TTKM) for software developers. In the first study, initial scale items were developed using the repertory grid technique and content analysis. In Study 2, supplied repertory grids were administered to novices and experts to establish differential items, and Study 3 validated the TTKM on a sample of 48 industrial software development teams. In developing the TTKM we explored the relationships between tacit knowledge, explicit job knowledge and social interaction and their effect on team performance as measured by efficiency and effectiveness. In addition we assess the implications for managing software development teams and increasing team performance through social interaction.