Autism spectrum disorder (ASD) is a severe neurodevelopmental disorder that affects 1 in 59 children. Although many early behavioral symptoms of ASD are observable by the first birthday, age of diagnosis remains problematically high, with many children not diagnosed until 3–5 years of age or even later. Appropriate screening and early diagnosis can facilitate early intervention, which has been shown to improve long-term outcomes in children with ASD. This chapter focuses on best practices in screening for and diagnosing ASD in young children. We begin with a discussion of the earliest signs of ASD and early screening practices and challenges. Then, we provide a thorough overview of gold standard diagnostic procedures and measures. We conclude with a discussion on several important considerations involved in diagnosing young children with ASD, including the role of intellectual disability, common comorbid psychiatric disorders, sex differences, and cultural and socioeconomic considerations.