As ASD is a lifelong developmental disorder, there is a need to examine behavioral trajectories from the emergence of symptoms to long-term outcomes. This panel aims to shed light on the developmental continuum of ASD based on data from four independent cohorts followed during different developmental stages from infancy to middle adulthood. The first presentation examines trajectories of directly observed ASD symptoms in a cohort of infants and toddlers seen approximately every 1-6 months from 1-3 years. The second presents ASD symptom trajectories based on parent interviews repeated on five occasions between 2-18 years and their relation to parent-reported language. The third uses multiple assessment modalities to demonstrate changes in ASD and mental health symptoms, as well as cognitive and adaptive functioning from 12-16 years in a population-based sample. The last compares child and middle-adulthood ASD symptoms, IQ and adaptive behavior in a 30-year follow up study of individuals initially assessed between 2-16 years. By providing insight into stability and change across different developmental periods, these studies highlight methodological challenges to assessment of individuals with ASD. Taken together, findings underscore the need for careful diagnostic, behavioral and intellectual assessment capturing variability in trajectories and outcomes over time.