Governments use statutes, regulations, and policies, often in innovative ways, to promote health and safety. Organizations outside government, from private schools to major corporations, create rules on matters as diverse as tobacco use and paid sick leave. Very little of this activity is systematically tracked. Even as the rest of the health system is working to build, share, and use a wide range of health and social data, legal information largely remains trapped in text files and pdfs, excluded from the universe of usable data. This article makes the case for the practice of policy surveillance to help end the anomalous treatment of law in public health research and practice. Policy surveillance is the systematic, scientific collection and analysis of laws of public health significance. It meets several important needs. Scientific collection and coding of important laws and policies creates data suitable for use in rigorous evaluation studies. Policy surveillance addresses the chronic lack of readily accessible, nonpartisan information about status and trends in health legislation and policy. It provides the opportunity to build policy capacity in the public health workforce. We trace its emergence over the past fifty years, show its value, and identify major challenges ahead.