Adaptive Behavior skills are the conceptual, social, and practical skills that individuals learn to be able to function in their everyday lives (AAIDD, 2008). Measuring adaptive behavior is a way to summarize the effectiveness with which individuals meet the standards of personal independence and social responsibility expected for their age and cultural group. This paper discusses the history and development of adaptive behavior as a construct, its measurement, and its relationship to intelligence. Previous research has examined the relationship between adaptive and intellectual functioning; this study investigates adaptive performance among children with disabilities while controlling for the influence of intellectual level. Children with autism, specific learning disabilities, and mental retardation were studied to determine how they fared in the adaptive subdomains of communication, socialization, and activities of daily living. Data for the study were gathered by reviewing archives from special education records in a large, urban school district. Results indicated a positive and moderate relationship between intelligence and adaptive behavior, but only in the autism group. The groups differed in their performance on the subdomains of adaptive behavior; however, the pattern of adaptive skills for each diagnostic group was unique. Children with autism were found to have deficits in socialization, children with learning disabilities were found to have deficits in communication, and children with mental retardation showed deficits in all domains. These patterns held up even when IQ was controlled; however, the groups no longer differed on communication skills, suggesting that IQ is most strongly related to communication. Finally, the study revealed that full scale IQ, activities of daily living, and communication skills discriminate mental retardation from the other groups while socialization skills discriminate autism from the other groups. Implications of these findings are discussed relative to assessment practices, differential diagnosis, program development, and progress monitoring.