This article addresses two major areas concerned with the evolving understanding of the construct of intellectual disability. The first part of the article discusses current answers to five critical questions that have revolved around the general question, "What is Intellectual Disability?" These five are what to call the phenomenon, how to explain the phenomenon, how to define the phenomenon and determine who is a member of the class, how to classify persons so defined and identified, and how to establish public policy regarding such persons. The second part of the article discusses four critical issues that will impact both our future understanding of the construct and the approach taken to persons with intellectual disability. These four critical issues relate to the conceptualisation and measurement of intellectual functioning, the constitutive definition of intellectual disability, the alignment of clinical functions related to diagnosis, classification, and planning supports, and how the field resolves a number of emerging epistemological issues. (Contains 3 tables.)