THE COMMON LAW DOCTRINE THAT A DEFENDANT MUST BE MENTALLY COMPETENT TO STAND TRIAL HAS BEEN A PART OF CRIMINAL LAW SINCE BEFORE THE 17TH CENTURY. THIS DOCTRINE HAS BEEN APPLIED WITH INCREASING FREQUENCY, UNTIL IN THE 1960'S ABOUT HALF OF ALL PERSONS IN MENTAL INSTITUTIONS WERE THERE BECAUSE THEY HAD BEEN FOUND INCOMPETENT TO STAND TRIAL. THE RATIONALE BEHIND COMPETENCY DETERMINATIONS AND THE PROCEDURES USED TO ARRIVE AT THEM ARE EXAMINED IN DETAIL. VARIOUS JUDICIAL DECISIONS AND STATUTORY PROVISIONS ARE EXAMINED. IT HAS BEEN HELD THAT A COMPETENCY HEARING IS MANDATORY FOR DUE PROCESS, ALTHOUGH THIS MAY BE SIMPLIFIED, PARTICULARLY IF SUCH SIMPLIFICATION IS NECESSARY FOR THE ACCUSED TO UNDERSTAND THE PROCEEDINGS. THIS HEARING MAY COMMIT THE ACCUSED TO A MENTAL HOSPITAL FOR ONLY AS LONG AS IS NECESSARY TO DETERMINE THE PROBABILITY THAT COMPETENCY WILL BE REGAINED. IF RECOVERY IS NOT FORESEEABLE, AS MAY BE THE CASE WITH SEVERE MENTAL RETARDATION, THE DEFENDANT MUST EITHER BE RELEASED OR CIVILLY COMMITTED. THE DUE PROCESS ELEMENTS WHICH MUST BE PRESENT DURING A COMPETENCY TEST ARE LISTED. THE ROLE OF VALUE JUDGMENTS IN THE INTERPRETATION OF COMPETENCY TESTS IS EXAMINED. FACTORS INFLUENCING SUCH JUDGEMENTS ARE DISCUSSED, INCLUDING THE ABILITY OF THE DEFENDANT TO PUT ON A CONVINCING PERFORMANCE. PRO SE COMPETENCY IS ALSO DISCUSSED. IT IS SUGGESTED THAT LEGISLATURES REPLACE VAGUE TERMS SUCH AS 'INSANITY' WITH MORE DISTINCTIVE TERMS, SUCH AS 'INSANITY AT THE TIME OF THE ACT,' 'INCOMPETENCY TO STAND TRIAL,' AND 'CIVIL COMMITABILITY.' COURTS SHOULD BE MORE SPECIFIC IN SPELLING OUT THE PURPOSE OF THE COMPETENCY TEST. ERRORS SHOULD BE MINIMIZED BY DEFINING THE FUNCTIONS WHICH A DEFENDANT WILL BE EXPECTED TO PERFORM DURING A TRIAL AND EXAMINING HIS OR HER ABILITY TO FULFILL EACH OF THESE FUNCTIONS. THIS WILL MINIMIZE THE VAGUENESS ASSOCIATED WITH THE PRESENT TERM 'COMPETENCY TO STAND TRIAL' AND REDUCE OVERAPPLICATION OF THE DOCTRINE. THE DISCUSSION IS FOOTNOTED. (GLR)