To develop a valid, reliable and feasible teamwork assessment measure for emergency resuscitation team performance.Generic and profession specific team performance assessment measures are available (e.g. anaesthetics) but there are no specific measures for the assessment of emergency resuscitation team performance.(1) An extensive review of the literature for teamwork instruments, and (2) development of a draft instrument with an expert clinical team. (3) Review by an international team of seven independent experts for face and content validity. (4) Instrument testing on 56 video-recorded hospital and simulated resuscitation events for construct, consistency, concurrent validity and reliability and (5) a final set of ratings for feasibility on fifteen simulated ‘real time’ events.Following expert review, selected items were found to have a high total content validity index of 0.96. A single ‘teamwork’ construct was identified with an internal consistency of 0.89. Correlation between the total item score and global rating (rho 0.95; p<0.01) indicated concurrent validity. Inter-rater (k 0.55) and retest reliability (k 0.53) were ‘fair’, with positive feasibility ratings following ‘real time’ testing. The final 12 item (11 specific and 1 global rating) are rated using a five-point scale and cover three categories leadership, teamwork and task management.In this primary study TEAM was found to be a valid and reliable instrument and should be a useful addition to clinicians’ tool set for the measurement of teamwork during medical emergencies. Further evaluation of the instrument is warranted to fully determine its psychometric properties.