Given growing global interest in addressing lead poisoning, there is an urgent need to better understand the major sources of lead exposure in different settings by increasing the volume and quality of related research. Tailored to a policy audience, this paper offers a state-of-the-science review of methods for doing so. First, it provides background for understanding lead exposure—that is, how and why lead enters the environment, how it behaves once there, and how environmental lead results in elevated blood lead levels for humans. Second, it outlines basic principles for source assessment, including a discussion of objectives and terminology. Third, it details the range of methods available for understanding the sources of lead exposure and describes how such methods can be used in combination to understand major sources of lead exposure in a given context. Finally, it considers major policy-relevant gaps in the literature base about sources of global exposure and offers suggestions for a research agenda in this space.