Osteochondritis dissecans (OCD) of the knee remains a relatively common and poorly understood pediatric and adolescent knee condition. Both conservative and surgical treatments have major impact on the lives of young active, athletic patients with knee OCD. OCD has been recently redefined as a "focal, idiopathic alteration of subchondral bone with risk for instability and disruption of adjacent articular cartilage that may result in premature osteoarthritis." The 2011 American Academy of Orthopedic Surgeons Clinical Practice Guidelines found limited evidence for all aspects of the treatment of knee OCD. The multicenter study group Research in Osteochondritis dissecans of the Knee (ROCK) was formed to advance the understanding and treatment of this condition. This article will review our current understanding of the pathophysiology, treatment options, and outcomes of OCD of the knee, with a focus on the past, present, and future research including the work of the ROCK study group.