Teamwork training is critical for patient safety and has been advocated for widespread application in many settings. A key challenge for evaluating teamwork training is measurement. Despite much effort, the team performance instruments reported thus far suffer from a variety shortcomings that prevent their wide application in assessing teams in real settings. Based on review of video recorded trauma team activities in real patient care, a multi-disciplinary research team developed an instrument based on observable behaviors (UMTOP). A set of video clips were reviewed by 6 subject matter experts who were requested to provide “descriptors” about the observed team activities. The 167 collated descriptors were combined to a reduced list, which was then sent to the subject matter experts for revision. The revised list was then categorized into 5 areas of team performance (task and clinical performance, leadership organization, teamwork organization, social environment, sterile precaution). UMTOP was developed to be a tradeoff among four criteria: ease of use, reliability, usefulness for team performance feedback, and speed of scoring. An initial assessment of reliability was conducted with surgeon and nursing reviewers.