Previous conceptualizations of autism have suggested that symptoms are evident either early in the first year of life or later in the second year, after a loss of previously acquired skills. New research suggests, however, that these two patterns do not capture all the different ways autism can emerge. For example, some children show a developmental plateau marked by failure to progress, while other children display mixed features, with both early delays and later losses evident. This article reviews the literature on autism onset, discusses problems with the traditional ways in which onset has been conceptualized, and provides recommendations for future research. We suggest that onset is better thought of as a dimensional process rather than dichotomous categories.