Systematic reviews are generating valuable scientific knowledge about the impact of public health laws, but this knowledge is not readily accessible to policy makers. We identified 65 systematic reviews of studies on the effectiveness of 52 public health laws: 27 of those laws were found effective, 23 had insufficient evidence to judge effectiveness, 1 was harmful, and 1 was found to be ineffective. This is a valuable, scientific foundation-that uses the highest relevant standard of evidence-for the role of law as a public health tool. Additional primary studies and systematic reviews are needed to address significant gaps in knowledge about the laws' public health impact, as are energetic, sustained initiatives to make the findings available to public policy makers.