Osteochondritis dissecans (OCD) has been known for more than 100 years. Several reasons have been suggested as the major cause for developing OCD: trauma/microtrauma, ischaemic or genetic reasons. OCD is primarily located in the knee. In adolescent patients with knee symptoms, X-ray of the affected knee and often a subsequent MRI scan is indicated. Depending on the findings, an arthroscopy can be performed. Conservative treatment can be quiet regime or bracing. Operative treatment can be transchondral/retrograde drilling for the in situ lesion or fixation of the unstable lesion.