Ankle sprains are the most common musculoskeletal injury, with an incidence of 30,000 per day in the United States; 40% of all athletic injuries involve the ankle. Most ankle sprains involve the lateral ligament complex and are caused by an inversion force on a plantarflexed foot. In 1965, Freeman hypothesized that injury to the mechanoreceptors of the ankle joint was the main factor for continued symptoms after ankle sprain. Despite the large number of ankle sprains each year, however, most individuals do not develop chronic ankle instability. Those who do generally have two distinct presentations. Patients with mechanical instability complain of giving way and have documented pathologic hypermobility of the tibiotalar joint. Individuals with functional instability present with subjective complaints of an unreliable ankle but lack any demonstrable radiographic signs of instability. Significant research in recent years indicates that patients with either acute lateral ligament tears or chronic functional instability are best managed with a bracing and rehabilitation program, as substantiated by the 2002 Cochrane Database review by Kerkhoffs et al. Controversy still exists, however, regarding the optimal treatment of individuals with chronic mechanical ankle instability. What has become clear is that prevention of lateral ankle instability is mandatory, because the relationship between chronic instability and late sequelae, such as arthritic progression has become more obvious, although further Level I (Table 1) outcomes research are necessary to define these issues.