ABSTRACT Injury is the leading cause of loss of independence among those over the age 65, resulting in over 3 million Emergency Department (ED) visits, 800,000 hospitalizations, and greater than $50 billion in costs each year. Guideline-concordant triage of trauma patients – rapid identification of those with severe injuries and transfer to trauma centers – decreases mortality by 10 to 25%, reduces loss of independence, and diminishes pain at one year. Despite this evidence, under-triage persists (incidence: 50%), particularly among those over the age of 65 (incidence: 80%). Causes of non-compliance with clinical practice guidelines varies among physicians. Moreover, exposing physicians to inappropriate interventions can have serious unintended consequences. To address the implementation gap, we therefore propose to develop a novel serious game (an applied video game) to measure physician behavior in trauma triage and to identify sources of non-compliance. In Aim 1, we will build the game using an established, user-centered design process that involves formative research with stakeholders (n=20), prototype development, and then calibration through iterative play-testing with a national convenience sample of physicians (n=30). In Aim 2, we will establish the initial measurement properties of the game by recruiting a local convenience sample of physicians (n=100), auditing their charts, and conducting semi-structured interviews so that we can assess internal consistency reliability, re-test reliability, criterion validity, and convergent construct validity. This proposal will deliver a tool that can be used to inform the selection of interventions in future efforts to increase the implementation of guidelines in trauma triage, and thereby to improve patient outcomes after injury.