Significant intra-articular knee injuries, including tears of the anterior cruciate ligament (ACL), are now being recognized more frequently in skeletally immature patients. Previously reported data on ACL tears in this age group are sparse, and studies have been of limited quality. Improvements in diagnostic techniques (e.g., physical examination signs, arthrometric testing, magnetic resonance imaging, and arthroscopy) have facilitated identification of such injuries. Hemarthrosis must be regarded as a herald of a major intra-articular injury. Surgical reconstruction options vary according to the specific diagnosis and the stage of maturity, and the available options for the very skeletally immature patient are limited. Therefore, treatment must be predicated on assessment of maturity, as determined on the basis of chronologic, radiologic, and physiologic criteria. An ACL injury in this age group is not a surgical emergency; therefore, time for discussion with the patient and his or her parents is available, so that all appropriate options can be considered.