This article examines the relationship of police and child protective services (CPS) coinvolvement to the outcomes of child maltreatment investigations. It reviews practice and empirical literature and conducts a secondary analysis of a national CPS data set. Most sources argue that coordination of the two agencies improves investigations and benefits children and families. Yet, sources also report friction between these agencies, interference with each other's job, and concerns that police involvement increases child removal. In the CPS case data, allegations were more likely to be judged credible when police also investigated and families were also more likely to receive various services. For neglect cases, multi-disciplinary decision making, but not police involvement per se, was linked to child removal. Across studies, police do not appear to hinder CPS effectiveness and may actually promote it. Their investigations should be coordinated in every community.