AUTISM IS A RARE DISORDER CHARACTERIZED BY A DELAY IN SPEECH AND LANGUAGE DEVELOPMENT AND FAILURE TO DEVELOP INTERPERSONAL RELATIONS. ITS CAUSE IS UNKNOWN AND IT IS RARE. CEREBRAL PALSY IS A BRAIN-CENTERED SET OF DISORDERS CHARACTERIZED BY SEIZURES AND CONVULSIONS. HOWEVER, MOST DEVELOPMENTALLY IMPAIRED OFFENDERS THAT OFFICERS WILL ENCOUNTER WILL BE MENTALLY RETARDED, AS THIS IS MORE PREVALENT THAN THE OTHERS. THE MENTALLY RETARDED ARE SUBAVERAGE IN GENERAL INTELLECTUAL FUNCTIONING AND IMPAIRED IN MATURATION, LEARNING, AND SOCIAL ADJUSTMENT. INTELLIGENCE QUOTIENTS MAY RANGE FROM 69 ON DOWN, AND MILDLY OR MODERATELY RETARDED PERSONS CAN DEVELOP SOCIAL AND VOCATIONAL SKILLS. MOST OF THE MENTALLY RETARDED ARE NOT INSTITUTIONALIZED. THE POLICE MAY DISTINGUISH A MENTALLY RETARDED PERSON BY HIS INAPPROPRIATE DRESS, AWKWARDNESS IN BODY, SLOWNESS, AND INCLINATION TO SOCIALIZE WITH YOUNGER PEOPLE. THE MENTALLY RETARDED PERSON WIL COME INTO CONTACT WITH THE POLICE WHEN HE IS LOST OR CONFUSED, VICTIMIZED, OR BREAKING THE LAW; HOWEVER, THE MENTALLY RETARDED ARE NO MORE LIKELY TO BE CRIMINALS THAN OTHERS. SOME COMMON MISCONCEPTIONS ABOUT MENTALLY RETARDED PERSONS ARE DISCUSSED, AND CLUES TOWARD IDENTIFYING THEM AND DISTINGUISHING THEM FROM THE DEAF, MENTALLY ILL, OR CEREBRAL PALSIC ARE PROVIDED. MEANS OF TELLING THE DEGREE OF MENTAL RETARDATION ARE IDENTIFIED, ALSO. A LIST OF AGENCIES DEALING WITH THE MENTALLY RETARDED IN SOUTH CAROLINA IS GIVEN. REFERENCES AND A MAP ARE INCLUDED.