Acrophobia is an extreme or irrational fear of heights. It belongs to a category of specific phobias, called space and motion discomfort that share both similar etiology and options for treatment.

Acrophobia can be dangerous, as sufferers can experience a panic attack in a high place and become too agitated to get themselves down safely.

"Vertigo" is often used, incorrectly, to describe the fear of heights, but it is more accurately described as a spinning sensation, which may be caused by looking down from a high place, as well as by some other stimuli. Vertigo is qualified as height vertigo when referring to dizziness triggered by heights.

Traditionally, acrophobia has been attributed, like other phobias, to conditioning or a traumatic experience involving heights. Recent studies have cast doubt on this explanation; fear of falling, along with fear of loud noises, is one of the most commonly suggested inborn or non-associative fears. The newer non-association theory is that fear of heights is an evolved adaptation to a prehistory where falls posed a significant danger. The degree of fear varies and the term phobia is reserved for those at the extreme end of the spectrum. It has been argued by researchers that fear of heights is an instinct found in many mammals, including domestic animals and human beings. Experiments using what are known as "visual cliffs" have shown human infants and toddlers, as well as other animals of various ages, to be reluctant in venturing onto a glass floor with a view of a few meters of apparent fall-space below it. While an innate cautiousness around heights might be helpful for survival, an extreme fear can interfere with the activities of everyday life, such as climbing up a flight of stairs or a ladder.

A possible contributing factor is dysfunction in maintaining balance. In this case the anxiety is both well founded and secondary. The human balance system integrates proprioceptive, vestibular and nearby visual cues to reckon position and motion. As height increases visual cues recede and balance becomes poorer even in normal people. However most people respond by shifting to more reliance on the proprioceptive and vestibular branches of the equilibrium system.

An acrophobic, on the other hand, continues to overrely on visual signals whether because of inadequate vestibular function or incorrect strategy. Locomotion at a high elevation requires more than normal visual processing. The visual cortex becomes overloaded resulting in confusion. Some proponents of the alternative view of acrophobia warn that it may be ill-advised to encourage acrophobics to expose themselves to height without first resolving the vestibular issues. Research is underway at several clinics.