College undergraduates were asked the degree to which they believed certain behaviors and characteristics are present in mentally retarded and mentally ill persons. Comparisons of responses showed that subjects clearly differentiated the concepts, although several areas overlapped. Mental retardation was characterized by physical stigmata and brain damage, developmental delays, and cognitive deficits; mental illness, by emotional lability due to environmental, hereditary, or mixed factors. How knowledge of people's perceptions of these disorders is essential for a more complete understanding of reactions to group homes, mainstreaming, and other "normalized" placements was discussed.