Medial instability is suspected on the basis of a patient's ankle feeling like it is "giving way," especially medially, when walking on uneven ground, downhill, or down stairs, pain at the anteromedial aspect of the ankle, and sometimes pain in the lateral ankle, especially during dorsiflexion of the foot. A history of a chronically unstable feeling that is manifested by recurrent injuries with pain, tenderness, and sometimes bruising over the medial and lateral ligaments, is considered to indicate combined medial and lateral instability that is believed to result in rotational instability of the talus in the ankle mortise. Pain on the medial gutter of the ankle and a valgus and pronation deformity of the foot are hallmarks of the disorder. The deformity typically can be corrected by the activation of the posterior tibial muscle. In contrast to stress radiographs, arthroscopy is a helpful diagnostic tool in verifying medial instability; it proved that the lateral ankle ligaments also can be involved. The treatment for symptomatic medial instability of the ankle might include reconstruction of all involved ligaments at the medial, and, if necessary, the lateral ankle. In the case of progressed foot deformity or bilateral long-standing valgus and pronation deformity of the foot, an additional calcaneal lengthening osteotomy might be considered. A classification of the instability into three types has been helpful for determining surgical treatment and the after treatment. This treatment concept provides high patient satisfaction and reliable clinical results.