Publisher Summary This chapter reviews the growth and development of the field of assessment in intellectual disability (ID). The history of the assessment of people with ID goes back to the roots of modern day research and practice and to the roots of many social policies. The development of IQ tests grew out of the need in the early 1900s to differentiate people with ID, to predict school success better. Out of this work emerged a new definition of intelligence—the idea of general intelligence (g) and the concept of intelligence consisting of several primary abilities. As federal entitlements to services for people with ID grew, the need for standardized measures on work performance, social skills, language development, quality of life (QOL), family adaptation, psychopathology, and efficacy of psychopharmacology developed in the latter half of the twentieth century. Another movement, which greatly affected assessment of people with ID during this period, was the advancement of the field of applied behavior analysis (ABA) in the 1960s. Currently, the possibility of multimodal assessment using concomitant psychophysiological and neuroimaging assessment is being demonstrated to great advantage in the field of learning disabilities and is considered in ID. Interdisciplinary assessment is increasingly becoming recognized as an important part of assessing people with mental retardation. Behavioral and psychometric assessments have an integral role to play in these hopes and aspirations.