While most in the scientific community are of the opinion that the composition of fine particulate matter (PM2.5) is an important driver of resultant health effects, there is still some degree of uncertainty regarding those component(s) considered to be most harmful. Reviews of the subject from several perspectives have been published, but to our knowledge a comprehensive review of the epidemiological and toxicological literature related to long-term exposure to PM2.5 components does not exist. We reviewed published epidemiological studies that were of a cohort design, included at least one PM component as well as PM2.5 mass, and included quantitative analysis to relate health outcomes to individual components. Toxicological studies were included if they were ≥ 5 months in duration, and either included at least one PM component as well as PM mass or focused on a specific PM or emissions type. Overall, we find that epidemiological and toxicological evidence for long term effects of PM components is limited, in contrast to the short term literature which is more plentiful. Epidemiological literature suggests that a number of components are associated with health effects, and that no component is unequivocally not so associated. Toxicological studies that can more easily identify potentially causal components are Downloaded by [Universite Laval] at 06:38 03 March 2015 Accepted Manuscript 2 generally limited to long term studies using concentrated ambient particles (CAPs), of which few long term studies exist. Epidemiological study designs that utilize existing monitoring data routinely collected by the United States Environmental Protection Agency would be valuable additions to the literature, as would novel toxicological studies that incorporate innovative designs to separate components or groups of components, such as denuders, filtration, or other approaches. From a policy perspective, it is important to more comprehensively investigate this issue so that if particular constituents are determined to be more potent in inducing health effects, their source(s) can be controlled.