This project aims to map the early developmental trajectories of young children with autism. This will be done in three ways: by longitudinally following young children at elevated likelihood for autism (siblings of an older child with autism, very and extremely premature children) from the age of 5 months into toddlerhood and preschool age; by following young children diagnosed with autism from the age of 2 years to 5 years and comparing them with typically developing children, children with ADHD, and children with an intellectual developmental disorder; by implementing a prodromal intervention for children at elevated likelihood for autism from the age of 10 months. The project will not only lead to a better understanding of autism but also to more reliable early diagnostics and more effective guidance and intervention.