Although reductions in lead (Pb) exposure for the U.S. population have resulted in lower blood Pb levels over time, epidemiological studies continue to provide evidence of health effects at lower and lower blood Pb levels. Low-level Pb was selected for evaluation by the National Toxicology Program (NTP) because of (1) the availability of a large number of epidemiological studies of Pb, (2) a nomination by the National Institute for Occupational Safety and Health for an assessment of Pb at lower levels of exposure, and (3) public concern for effects of Pb in children and adults. This evaluation summarizes the evidence in humans and presents conclusions on health effects in children and adults associated with low-level Pb exposure as indicated by less than 10 micrograms of Pb per deciliter of blood (< 10 microg/dL). The assessment focuses on epidemiological evidence at blood Pb levels < 10 microg/dL and < 5 microg/dL because health effects at higher blood Pb levels are well established. The NTP evaluation was conducted through the Office of Health Assessment and Translation (OHAT, formerly the Center for the Evaluation of Risks to Human Reproduction) and completed in April of 2012. The results of this evaluation are published in the NTP Monograph on Health Effects of Low-Level Lead. The document and appendices are available at http://ntp.niehs.nih.gov/go/evals. This document provides background on Pb exposure and includes a review of the primary epidemiological literature for evidence that low-level Pb is associated with neurological, immunological, cardiovascular, renal, and/or reproductive and developmental effects. The NTP Monograph presents specific conclusions for each health effect area. Overall, the NTP concludes that there is sufficient evidence that blood Pb levels < 10 microg/dL and < 5 microg/dL are associated with adverse health effects in children and adults. This conclusion was based on a review of the primary epidemiological literature, scientific input from technical advisors that reviewed pre-public release drafts of each chapter summarizing the evidence for specific health effects associated with low-level Pb, public comments received during the course of the evaluation, and comments from an expert panel of ad hoc reviewers during a public meeting to review the Draft NTP Monograph on November 17-18, 2011 (http://ntp.niehs.nih.gov/go/37090.