Juveniles' competency to participate in delinquency proceedings has received increased attention in recent years. Developmental incompetence, whereby juveniles' incompetency is based upon their immaturity, as opposed to a mental disorder or developmental disability, is an evolving and important aspect of this area of law. The following paper reviews theories used to support the notion of developmental incompetence, as well as the extant empirical research on juveniles' competency-related abilities. Using a LexisNexis search, statutory and case laws pertaining to juvenile competency were identified across the 50 States and the District of Columbia. Only six States clearly allow developmental incompetence, whereas 17 have laws that do not include developmental immaturity as an acceptable basis of incompetence in juvenile courts. Developmental incompetence is likely to affect a relatively small proportion of juvenile cases, but has important implications for juvenile forensic practice. Recommendations are offered for forensic practitioners conducting this type of evaluation. Abstract published by arrangement with John Wiley & Sons.