This chapter provides a detailed overview of the assessment of Autism Spectrum Disorder (ASD) across the lifespan, beginning with a definition of ASD, its core characteristics and symptoms, and the importance of early identification in shaping intervention outcomes. The chapter emphasizes a multidisciplinary team approach to assessment, incorporating expertise from various fields to ensure a comprehensive evaluation. A detailed review of both generalized and autism-specific assessment measures is provided, covering tools used across the lifespan from early childhood through adulthood, as well as adaptive functioning and executive function (EF) measures. Tailoring the assessment process to the individual’s needs and profile is highlighted, ensuring the use of appropriate methods for each unique presentation. The chapter also explores differential diagnosis and co-occurring conditions, including developmental, communication, and anxiety disorders, to aid clinicians in distinguishing ASD from other conditions and understanding the broader clinical context. Modern research on racial and ethnic bias in ASD diagnosis is discussed, with an emphasis on addressing disparities and promoting equitable care. The female phenotype of ASD and the barriers to diagnosis faced by women and gender nonconforming individuals are also examined, highlighting the need for increased clinician awareness of diverse presentations. The chapter concludes with case vignettes that illustrate the factors influencing the selection of an appropriate testing battery and provide practical examples of how demographic considerations can inform the design of the diagnostic interview and the interpretation of testing results.