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Epidemiology is a discipline of medicine dedicated to the study of patterns related to health and disease processes in populations. As in the rest of medicine, observations of the frequency of diseases or their determinants are the basis for prevention and treatment measures. More recent applications of epidemiology in psychiatry have contributed to a better understanding of neurodevelopmental disorders in children. Thus, developmental epidemiology is the study of developmental processes and their determinants approached through multiple biological, environmental, and psychological dimensions. In this article, we illustrate the contributions of developmental epidemiology to the understanding of risk and protective factors of early developmental trajectories in autism spectrum disorders. We describe a cohort study (ELENA) in which 900 children with a confirmed diagnosis of ASD are followed-up prospectively for 6 years. The objective of this study is to describe the developmental trajectories of these children and their biological, environmental, and psychological determinants.