Osteochondritis dissecans (OCD) is a common entity in both the juvenile and adult populations, with an incidence of 3 to 6/10,000 in adults. Much of the early literature grouped juvenile and adult osteochondritis dissecans, osteochondral fracture, and accessory ossification into the same category. Conclusions were then drawn on the combined group. Nonetheless, this is a diverse group. This review discusses only OCD. There have been multiple etiological theories of OCD, ranging from trauma to ischemia to accessory centers of ossification and to genetics. It is evident that the true etiology is probably multifactorial. Bone scan, computed tomographic scan, and magnetic resonance imaging advances have enhanced the physician's ability to make the diagnosis of osteochondritis dissecans as well as to stage operative intervention. There is a vast difference between juvenile and adult OCD, as seen in the natural history, prognosis, and treatment options. In general, the juvenile patients have better results overall. The indications for operative intervention for these juvenile patients are a nonhealing attached fragment, fully or partially detached lesions of the articular surface, and loose bodies. Nonoperative treatment in the adult patient has been shown to accelerate degenerative arthritis, which involves all 3 compartments of the knee. Therefore, symptomatic lesions and loose bodies comprise the surgical indications for adult OCD. An understanding of this disease process will help the physician optimize the patient's results.