The last decade has witnessed the rapid growth of a body of literature on environmental factors that alter risk for autism. Air pollution, household products, nutrition, maternal obstetric factors, medical interventions and medications, and infections have featured prominently, with particular focus on prenatal influences. This symposium will summarize the state-of-the-science 10 years ago, when few non-genetic etiologic clues had been uncovered. The bulk of the symposium will be devoted to describing major developments in methodology and results since then, providing depth on four classes of exogenous chemical and microbiologic exposures during gestation: air pollution, pesticides, nutrition, and infections. Dr. Marc Weisskopf will address what has been learned about air pollution, the challenges and advances in exposure assessment, recent convergent findings from multiple independent studies in different regions of the U.S. He will then present a novel analysis based on linking EPA modeled pollutant levels to residences near the time of birth of children born to Nurses Health Study-2 participants, including those with and without autism. Dr. Irva Hertz-Picciotto will summarize published and new evidence about insecticides, commonplace chemicals that are perhaps unique in having been designed specifically to damage living organisms. Potential mechanisms of neurotoxicity will be discussed along with epidemiologic studies covering both commercial (largely agricultural) and household applications of organochlorines, organophosphates, and pyrethroids, including new analyses from the CHARGE (CHildhood Autism Risks from Genes and Environment) Study. Dr. Rebecca Schmidt will describe the literature on maternal nutritional status in relation to autism, including several publications indicating a protective role of folate in the periconception, present evidence for gene-by-environment interactions with relatively common polymorphisms in the one-carbon metabolism pathway, and then report new data on prenatal iron status. Other aspects of diet will be touched on, and potential mechanisms such as a role for epigenetic alterations will also be discussed. Dr. Hjordis Atladottir will provide an overview of maternal infection and inflammation in autism etiology, beginning with possibly the earliest identified environmental factor, rubella, and tracing recent work on microbial exposures and inflammatory markers. This critical review will cover both epidemiologic and experimental results, characterize the methodologic limitations and pitfalls in work to date, and highlight where further work is needed. Together these four presentations will review our current knowledge base in regard to these four ubiquitous exposures, place this work in the context of the maternal-fetal interface, explain where the research is leading, highlight the challenges, and point to the directions that appear to be the most promising.