This chapter delves deeper into diagnostic challenges in adults with atypical neurodevelopment, especially women, for whom the correct neurodevelopmental diagnosis is often missed. It explores the biological and psychosocial factors that moderate the phenotypic expression of NDD, thus contributing to this problem. Other issues that make NDD diagnosis challenging in adults, including symptom masking, overlap, and comorbidity with other psychiatric illnesses, are also discussed. In addition to exploring illness-related factors that impact NDD diagnosis, the chapter also discusses pragmatic limitations of health care systems and psychiatry training in the United States that impede the diagnostic ability of clinicians primarily working with adult patients.