Special Report 16 summarizes the health effects of exposure to the 21 Mobile-Source Air Toxics (MSATs) defined by the 2001 EPA mobile-source rulemaking and critically analyzes the literature for a subset of seven MSATs (acetaldehyde, acrolein, benzene, 1,3-butadiene, formaldehyde, naphthalene, and polycyclic organic matter*). The report also assesses and summarizes research gaps and unresolved questions, as understood in the context of the current regulatory agenda. The Report focuses on MSATs for which mobile sources are a sizable source of human exposure and for which existing data suggest that health effects might be observed at concentrations approaching those found in ambient air. For each MSAT, the following questions are addressed: (1) To what extent are mobile sources a significant source of exposure to this MSAT? (2) Does this MSAT affect human health? (3) Does this MSAT affect human health at environmental conditions? Special Report 16 describes a review performed by a Special Panel with expertise in exposure assessment, toxicology, clinical medicine, and epidemiology. The Panel reviewed the peer-reviewed literature on MSATs, reached key conclusions, and made recommendations for future research. The completed report was reviewed by experts in a variety of fields and amended to reflect the useful comments and critiques.