Anxiety disorders are among the most prevalent mental disorders, but the subcategory of specific phobias has not been well studied. Phobias involve both fear and avoidance. For people who have specific phobias, avoidance can reduce the constancy and severity of distress and impairment. However, these phobias are important because of their early onset and strong persistence over time. Studies indicate that the lifetime prevalence of specific phobias around the world ranges from 3% to 15%, with fears and phobias concerning heights and animals being the most common. The developmental course of phobias, which progress from fear to avoidance and then to diagnosis, suggests the possibility that interrupting the course of phobias could reduce their prevalence. Although specific phobias often begin in childhood, their incidence peaks during midlife and old age. Phobias persist for several years or even decades in 10-30% of cases, and are strongly predictive of onset of other anxiety, mood, and substance-use disorders. Their high comorbidity with other mental disorders, especially after onset of the phobia, suggests that early treatment of phobias could also alter the risk of other disorders. Exposure therapy remains the treatment of choice, although this approach might be less effective in the long term than previously believed. This Review discusses the literature regarding the prevalence, incidence, course, risk factors, and treatment of specific phobias, and presents epidemiological data from several population-based surveys.