With the tragedies of school violence igniting calls for increased safety and security, one popular proposal is to use metal detectors in schools. To address questions about what impact such devices may have on day-to-day safety and on what happens during school violence events, this research brief summarizes what is known about metal detectors in schools and in other settings. The brief discusses: (1) The prevalence of metal detectors in schools; (2) The (in)effectiveness of metal detectors; (3) The cost associated with employing metal detectors; and (4) The potential impact on students and the learning environment as a whole. This is the third brief in the WestEd Justice and Prevention Research Center's "School Safety" series.