U.S. public policy has increasingly been conceived, debated, and evaluated through the lenses of politics and ideology. The fundamental question -- Will the policy work? -- too often gets short shrift or even ignored. A remedy is evidence-based policy--a rigourous approach that draws on careful data collection, experimentation, and both quantitative and qualitative analysis to determine what the problem is, which ways it can be addressed, and the probable impacts of each of these ways. Examples of how evidence informs good policy and lack of evidence can invite bad include health insurance coverage, education, sentencing policy, and redress for housing discrimination.