Fetal alcohol spectrum disorder (FASD) is the largest known cause of intellectual disability (ID), and forensic experts are often called upon to determine if a defendant with FASD qualifies for a diagnosis of ID. Whether such a diagnosis is made may depend upon the diagnosing expert’s choice of diagnostic manual: guidelines published by the American Association on Intellectual and Developmental Disabilities (now in its 12th edition [AAIDD-12]) or the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). Although both manuals use the same three diagnostic “prongs” (i.e., intellectual deficits, adaptive deficits, and developmental onset), there are substantial differences in the way all three prongs are assessed—differences that increased with the publication of AAIDD-12. In particular, AAIDD-12 uses a bureaucratic “disability” model, with narrow emphasis on a small number of quantitative indicators and limited opportunity for clinical integration, while DSM-5 (and the little-changed forthcoming DSM-5-TR) uses a medical “disorder” model, with flexible reliance on a broad array of indicators and opportunity for clinical integration. The origins and nature of these differences are explored, and an argument is made that compared to the AAIDD formulation, the DSM model provides a more valid basis for forensic diagnosis of ID in individuals with FASD.