The objective of this study was to evaluate the conditions of ankle stability and the morphological and/or lesional factors in sprains that determine when instability becomes chronic. It is based on a review of the literature and the data from the 2008 Sofcot symposium. The biomechanics of the ankle cannot be reduced to a simple flexion–extension movement with one degree of freedom as characterized by the talocrural joint: its function cannot be dissociated from the subtalar joint, allowing the foot to adapt to the ground surface. Functional stability is related to the combination of the particular biometry of the joint surfaces and a multiaxial ligament system. The bone morphology of the talus, shaped like a truncated cone, explains the potential instability in plantar flexion; the radii of curvature of the talar dome have a variable mediolateral distribution: most often the medial radius of curvature is inferior to the lateral radius of curvature (66%), sometimes equal (19%), or inverted (15%). Joint kinematics, combining rotation and slide, can therefore be modulated by the talar morphology, explaining the occurrence of at-risk ankles. Ligament stability relies on the organization in three parts of the lateral collateral ligament and the specific subtalar ligaments: the cervical and the talocalcaneal interosseous ligament. The different injury mechanisms are largely responsible for the sequence of ligament lesions: the most frequent is inversion. The first ligament stabilizers correspond to the cervical and anterior talofibular ligaments; the talocalcaneal ligament, by its oblique orientation, is solicited when there is a dorsal varus-flexion component. In chronic instability, these mechanisms explain the onset of associated lesions (impingement, osteochondral lesions, fibular tendon pathology), which can play a role in instability syndrome. Ligament lesions determine laxity, characteristic of mechanical instability. Functional instability goes along with proprioceptive deficiency. There are postural factors such as varus of the hindfoot that favor instability. Knowledge of all these factors, often associated, will provide a precise lesional assessment and treatment adapted to the instability.