Autism Spectrum Disorder (ASD) is a common neurodevelopmental disorder affecting up to 1 in 59 individuals. Clinical research in the last decade has shown that early and intensive interventions dramatically decrease the social deficits and learning difficulties faced by children affected with ASD. The first years of life may indeed represent a “window of opportunity”, when therapeutic interventions yield the most optimal long-term benefits. However, very little is known about early brain development in children with autism: when do the trajectories of brain development start to diverge between healthy children and those who will be later diagnosed with autism? What are the mechanisms by which early intensive intervention affects the brain development? As the autism spectrum is highly heterogeneous, can we distinguish different subgroups that will respond differently to treatment? Detailed information on the timing and nature of the neurodevelopmental disruption is critically required to improve therapeutic interventions for children with autism. To address these questions, we examine the trajectories of social, cognitive and cerebral development of children with ASD in the first years following diagnosis, to better understand how interventions can positively influence these trajectories. Our research protocol includes standardized clinical assessments, high-density EEG, MRI acquisitions and eye-tracking paradigms.