The aetiology of chronic functional lateral ankle instability is fairly well understood. Pathophysiological factors such as mechanical instability, proprioceptive deficit and peroneal muscle weakness have been demonstrated. Subtalar instability has been in focus during the last years as one of the possible factors behind chronic functional instability of the foot. The exact aetiology and the true incidence of subtalar ligament injuries remain unknown. Most subtalar ligamentous injuries probably occur in combination with injuries of the talo-tibial articulation. Subtalar instability can have the characteristics of chronic lateral instability or recurrent ankle sprains.Patients with chronic subtalar instability typically complain of ‘giving way’ symptoms and a history of recurrent sprains. Clinical examination including increased inwards rotation and forward displacement of the calcaneus may not be sufficient for the differentiation between ankle and subtalar instability. Radiographic imaging using stress radiographs may be necessary to assess subtalar instability. Subtalar instability can be defined as chronic functional instability with increased values of talar tilt and talo-calcaneal displacement as measured with standardised stress radiographs. Few authors have addressed the treatment of subtalar instability and the condition has not been clearly defined. Subtalar instability can be treated either with a tendon transfer or tenodesis procedure, such as the Chrisman-Snook or triligamentous tenodeses, or with an anatomic ligament reconstruction using the calcaneo-fibular, lateral talo-calcaneal and cervical ligaments combined with a reinforcement of the inferior extensor retinaculum. There have been no studies comparing anatomical and non-anatomical recon-structions and the long term results after ligamentous stabilisation are unknown.The focus of this article is on subtalar instability causing chronic functional ankle instability.