Emphasis on using evidence in federal policymaking and implementation is increasing, especially since the Foundations for Evidence-Based Policymaking Act of 2018 passed. It is important to find, build, share, and appropriately apply evidence. In this brief, we describe different types and sources of evidence, explore the purposes and potential uses of evidence, and discuss what makes good evidence in federal policy and practice. By understanding the many types, uses, and qualities of good evidence, federal policymakers and program managers can improve their policies and programs and—ultimately—social, economic, environmental, and other outcomes. We identify several key sources of evidence (administrative data, performance indicators, and evaluation and research) and explore the characteristics of good evidence. Finally, we discuss three discrete but complementary uses of evidence: (1) detect problems and inform where to focus attention and funding; (2) find what works, what works better, and the situational differences influencing effectiveness; and (3) increase use of better practices, and improve or reduce use of less effective or harmful ones.