This paper purpose is to suggest an in-depth approach to diagnose the causes and lesions associated with and consecutive to chronic ankle instability due to ankle collateral ligament laxity. The different therapeutic and medicosurgical options adapted to this diagnostic approach are identified. The diagnostic aim is to precisely locate the ligamentous injuries of the tibiofibular, subtalar, talar and calcanean system, to identify the predisposing factors such as the hindfoot morphology, and any lesions associated with chronicity: anterolateral impingement, fibular injury, osteochondral lesions of the talus dome and early osteoarthritis. Clinical tools are used in particular to identify areas of pain and for comparative analysis of mobility and laxity (ligament testing). There are also radiological tests, weight-bearing plain X-ray (stress X-ray), (alignment of the hind foot, with a Meary view [metal wire circling the heel], arthrosis), dynamic images to confirm and quantify laxity (manually, with a Telos device, with patient-controlled varus) and also more sophisticated techniques (ultrasound, CT arthrogramm, gadolinium enhanced MRI, MR arthrogramm) to identify ligament, tendon and cartilage damages. They are adapted to the lesions which have been identified in the diagnostic work-up: conservative first, to treat proprioceptive deficits (a new neuromuscular reprogramming technique which emphasizes muscle preactivation) and any static disorders (plantar orthotics); then surgical, to repair any collateral ligament (or sometimes subtalar) injury with three types of procedures: tightening the capsuloligamentous structures, ligament reconstruction with reinforcement (using the fibrous periosteum, the frondiform ligament (of Retzius) or tendinous reconstruction with the plantaris muscle, the peroneus tertius or even the calcanean tendon) and tendon tansfer procedures using all or part of the peroneus brevis (whole peroneus brevis and half peroneus brevis procedures). Any additional surgical procedures which may be indicated based on the results of the diagnostic work-up are performed at the same time as primary surgery when possible as needed (medial complex repair, calcaneal realignment osteotomies, talus osteochondral injuries debridment or fixation, anterior and posterior impingement suppression, tendon tears repair). The goal of this diagnostic and therapeutic approach is to stop the progression of laxity and to protect the ankle against degenerative arthritis, which is the main risk in these chronic conditions.