Concerns for student safety are a commonly cited reason for the presence of police in schools. But as the nation grapples with the appropriate roles and responsibilities of police more broadly, the time is right to shine a light on what we know about the effects of police in schools. This brief summarizes the research related to this question. This brief is part one of three in the "Police in Pennsylvania Schools" series, designed to inform conversations for Pennsylvania policymakers, school leaders, and other stakeholders. [For "School Policing in Pennsylvania: Prevalence and Disparities. Police in Pennsylvania Schools Series," see ED608042.]