A phobia is an intense irrational fear that often leads to avoidance of an object or situation.

Phobias are common (for example, fear of spiders, or arachnophobia; fear of heights, or acrophobia) and usually begin in childhood or adolescence.

Psychiatric nomenclature refers to phobias of specific places, objects, or situations as specific phobias. Fear of public speaking, in very severe cases, is considered a form of social phobia. Social phobias also include other kinds of performance fears (such as playing a musical instrument in front of others; signing a check while observed) and social interactional fears (for example, talking to people in authority; asking someone out for a date; returning items to a store). Individuals who suffer from social phobia often fear a number of social situations.

Although loosely regarded as a fear of open spaces, agoraphobia is actually a phobia that results when people experience panic attacks (unexpected, paroxysmal episodes of anxiety and accompanying physical sensations such as racing heart, shortness of breath).

The origin of phobias is varied and incompletely understood. Most individuals with specific phobias have never had anything bad happen to them in the past in relation to the phobia. In a minority of cases, however, some traumatic event occurred that likely led to the phobia. It is probable that some common phobias, such as a fear of snakes or a fear of heights, may actually be instinctual, or inborn. Both social phobia and agoraphobia run in families, suggesting that heredity plays a role. However, it is also possible that some phobias are passed on through learning and modeling.

Phobias occur in over 10% of the general population.