Autism spectrum disorder (ASD), or autism in short, is a complex neurodevelopmental disorder that appears in infancy and early childhood. During much of the twentieth century, autism was considered a rare disorder, but its diagnosis has increased in recent years, owing perhaps to a much broader definition and increased awareness of the condition. In 2014, the Centers for Disease Control and Prevention published its latest estimate of the prevalence of the disorder in the United States and identified 1 in 68 children (1 in 42 boys and 1 in 189 girls) as having autism. It is now clear that genes and environmental factors are implicated in autism. Although there is no cure for ASD, its symptoms can be treated with behavioral, psychosocial, and pharmacological interventions.