The clinical, social and financial burden of Autism Spectrum Disorder (ASD) is staggering. They are the most prevalent of the developmental disorders and their incidence is rising. However, the ASD phenotype variability is large, and ASD symptoms can manifest over a range of ages and to different degrees. In part for these reasons, the ASD clinical diagnosis is challenging and often is not made until 3-5 years of age. Thus, there remains an unmet need for a valid and reliable endophenotype which would facilitate ASD diagnosis early in life, enable efficient study of ASD risk factors, and eventually serve as a useful biomarker to inform the development of effective therapies and assess treatment response in future clinical trials. The overarching goal of this proposal is to explore the utility of transcranial magnetic stimulation (TMS) measures of brain plasticity as a novel neurophysiologic endophenotype in high- and low-functioning adults and children with ASD.