Educational and Developmental Psychologists are frequently required to assess a person for the presence or absence of an intellectual disability. The assessment is important as the availability of particular services is usually tied to a formal determination that an intellectual disability is present. A key element to deciding on the existence of an intellectual disability is the presence of a level of intelligence below a predetermined level. Although definitions of intellectual disability usually refer only to significant sub-average intellectual functioning, the actual cut-off score used to indicate this level of intellectual functioning has varied over time. Whatever that cut-off level is determined to be, there are technical difficulties that make assessment of intellectual functioning less than straightforward. These difficulties result from (a) standardized tests having different standard deviations, resulting in differing scores for cut-off levels based on standard deviation calculations, (b) different standardized tests giving different scores when administered to the same person, (c) less than perfect test reliability creating uncertainty about whether the measured intelligence corresponds with the actual intelligence of the individual being assessed, and (d) many people having impairments in sight, hearing, or motor function that make the standardized administration of tests impossible. It is recommended that educational psychologists be familiar with these issues so that they can make decisions that satisfy social justice objectives. Furthermore, it is argued that familiarity with these issues will help psychologists to defend their decisions when it is necessary.