The Rapid Assessment for Developmental Disabilities (RADD) was developed to address the challenges of measuring cognition in individuals with intellectual disabilities (ID). As opposed to individuals with ID, commonly used intelligence tests such as the Wechsler Intelligence Scale for Children (WISC) 1, the Wechsler Adult Intelligence Scales (WAIS) 2, and the Stanford-Binet (SB) 3 were designed for use with the general population 4. By design on these measures approximately 95% of the population have IQ scores that fall within two standard deviations (SD) from the mean (scores between 70 and 130; Fig. 12.1). Individuals with ID typically have IQ scores more than two SDs from the mean and they represent less than 2.5% of the total population, forming a statistically rare group. Commonly used intelligence tests were not designed to differentiate between the varying levels of ability among individuals at this lowest end of the intelligence spectrum. The RADD was developed to fill the need for measuring the cognitive abilities of individuals with IQ scores more than two standard deviations below the mean (scores below 70).