Advances in brain imaging technology over the last few decades have opened immense opportunities for identifying brain biomarkers. In older children and adults, functional magnetic resonance imaging (fMRI) studies have identified several neural systems consistently disrupted in autism, and structural imaging studies examining both gray matter and white matter, including diffusion tensor imaging studies, have noted abnormalities in regional size, white matter volume, and more recently, the growth trajectory of these regions (reviewed by Anagnostou and Taylor ). However, most of these studies have examined older children, adolescents, or adults who have an ASD diagnosis. Because of profound effects of experience on brain development, there is no way to determine whether brain differences observed in older children with autism precede the diagnosis or are a consequence of the diagnosis; that is, reduced social interest, social experience, and impaired development of linguistic competence may alter the trajectory of brain development. There are no data thus far indicating whether some of the brain differences found after diagnosis could also predict future diagnosis in infants who have yet to express behavioral abnormalities associated with autism.