In autism spectrum disorder (ASD), the field saw a move away from discrete, all-or-nothing categorical criteria and entities to recognizing a spectrum of presentations with new diagnostic criteria in the Diagnostic and Statistical Manual of Mental Disorders (5th Edition). Advances in genetics suggest a future in which our diagnostic boundaries, and even psychiatric taxonomy, may be redrawn yet again. At the same time, the prevalence of ASD is on a steady upward trajectory, and the search for explanations, particularly among environmental factors, is critically important.