Unlike other special education categories defined in U.S. law (Individuals with Disabilities Education Act), the definition of specific learning disability (SLD) has not changed since first proposed in 1968. Thus, although the operational definition of SLD has responded to new knowledge and understanding about the construct, the formal definition has remained static for 40 years, creating a schism between theory and practice. Using concepts gleaned from the scientific study of formal and operational definitions as well as the history of another special education category (i.e., mental retardation), in this article we demonstrate why change in the SLD definition is necessary. Finally, we propose a change in the SLD definition in federal regulations to redress the disconnect between theory and practice and restore integrity to the SLD field.