Autism spectrum disorder (ASD) is a highly complex neurodevelopmental condition that is accompanied by neuroanatomical differences on the macroscopic and microscopic level. Findings from histological, genetic, and more recently in vivo neuroimaging studies converge in suggesting that neuroanatomical abnormalities, specifically around the gray-white matter (GWM) boundary, represent a crucial feature of ASD. However, no research has yet characterized the GWM boundary in ASD based on measures of diffusion. Here, we registered diffusion tensor imaging data to the structural T1-weighted images of 92 adults with ASD and 92 matched neurotypical controls in order to examine between-group differences and group-by-sex interactions in fractional anisotropy and mean diffusivity sampled at the GWM boundary, and at different sampling depths within the superficial white and into the gray matter. As hypothesized, we observed atypical diffusion at and around the GWM boundary in ASD, with between-group differences and group-by-sex interactions depending on tissue class and sampling depth. Furthermore, we identified that altered diffusion at

Reliability and test-retest reproducibility of simple and advanced diffusion metrics were evaluated in healthy older adults. Advanced models such as Diffusion Kurtosis Imaging, White Matter Tract Integrity, Neurite Orientation Dispersion and Density Imaging and other multi-compartment methods have greater specificity but lower reliability and reproducibility than simple models such as DTI and spherical harmonic metrics. All models nevertheless had good or acceptable reproducibilty and reliability in certain cases. Particular care may be given when planning clinical research applications with advanced metrics, as greater sample sizes and/or improved data quality may be needed.