Though de-escalation is beginning to diffuse widely and rapidly in law enforcement, there is very little research on the effectiveness of de-escalation training. The Tempe SPI will develop and rigorously test a newly-designed violence de-escalation training, using a randomized controlled trial design. The training will be anchored in an innovative, peer-nomination process that asks all officers to identify the top de-escalators in the department, and then harnesses top de-escalators’ expertise to design the training. The Tempe SPI team and their Arizona State University research partners will conduct focus groups with top de-escalators, whereby officers will review body-worn camera footage of dozens of forceful and de-escalated police encounters and, using a sentinel events-type approach, will identify key principles of effective de-escalation. Once the principles of effective de-escalation have been identified, the top de-escalators, SPI team, and Tempe PD training instructors will convene to develop a unique de-escalation training, anchored in the key principles identified in the focus groups, the identified best practices from the available literature (Crisis Intervention and Mindfulness Training), and the collective expertise of the SPI team. The SPI team will test whether the de-escalation training produced measurable change in officers’ use of de-escalation tactics, led to fewer complaints and use of force, as well as whether the training led to enhanced officer and citizen perceptions of their encounters.