O. Ivar Lovaas (1927-2010) devoted nearly half a century to ground-breaking research and practice aimed at improving the lives of children with autism and their families. In the 1960s, he pioneered applied behavior analytic (ABA) interventions to decrease severe challenging behaviors and establish communicative language. Later, he sought to improve outcomes by emphasizing early intervention for preschoolers with autism, provided in family homes with active parental participation. His studies indicated that many children who received early intensive ABA made dramatic gains in development. Lovaas also disseminated ABA widely through intervention manuals, educational films, and public speaking. Moreover, as an enthusiastic teacher and devoted mentor, he inspired many students and colleagues to enter the field of ABA and autism intervention.