Acrophobia (from the , "ákron", meaning "peak, summit, edge" and , "phóbos", "fear") is an extreme or irrational fear or phobia of heights, especially when one is not particularly high up. It belongs to a category of specific phobias, called space and motion discomfort, that share both similar causes and options for treatment.

Most people experience a degree of natural fear when exposed to heights, known as the fear of falling. On the other hand, those who have little fear of such exposure are said to have a head for heights. A head for heights is advantageous for those hiking or climbing in mountainous terrain and also in certain jobs e.g. steeplejacks or wind turbine mechanics.

Acrophobia sufferers can experience a panic attack in high places and become too agitated to get themselves down safely. Approximately 2–5% of the general population suffers from acrophobia, with twice as many women affected as men.

Phobias are a common form of anxiety disorders and distributions are heterogeneous by age and gender. An American study by the National Institute of Mental Health (NIMH) found that between 8.7 percent and 18.1 percent of Americans suffer from phobias, making it the most common mental illness among women in all age groups and the second most common illness among men older than 25. Between 4 percent and 10 percent of all children experience specific phobias during their lives, and social phobias occur in one percent to three percent of children and adolescents.

A Swedish study found that females have a higher incidence than males (26.5 percent for females and 12.4 percent for males). Among adults, 21.2 percent of women and 10.9 percent of men have a single specific phobia, while multiple phobias occur in 5.4 percent of females and 1.5 percent of males. Women are nearly four times as likely as men to have a fear of animals (12.1 percent in women and 3.3 percent in men) — a higher dimorphic than with all specific or generalized phobias or social phobias. Social phobias are more common in girls than in boys, while situational phobia occurs in 17.4 percent of women and 8.5 percent of men.

The fear of flying may be created by various other phobias and fears:

- Fear of crashing, which in rare cases will cause death, is the most common reason for the fear of flying.

- Fear of closed in spaces (claustrophobia), such as that of an aircraft cabin

- Fear of heights (hypsophobia)

- Feeling of not being in control

- Fear of vomiting, where a person will be afraid that they'll have motion sickness on board, or encounter someone having motion sickness and have no control over it (such as escaping it)

- Fear of having panic attacks in certain places, where escape would be difficult and/or embarrassing (agoraphobia)

- Fear of hijacking or terrorism

Traditionally, acrophobia has been attributed, like other phobias, to conditioning or a traumatic experience. Recent studies have cast doubt on this explanation; a fear of falling, along with a fear of loud noises, is one of the most commonly suggested inborn or "non-associative" fears. The newer non-association theory is that a fear of heights is an evolved adaptation to a world 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. Researchers have argued that a fear of heights is an instinct found in many mammals, including domestic animals and humans. Experiments using 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 is helpful for survival, an extreme fear can interfere with the activities of everyday life, such as standing on a ladder or chair, or even walking up a flight of stairs.

A possible contributing factor is a 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, however, continues to over-rely 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.

Anticipatory anxiety of being out of control and overwhelmed can prevent a person from planning to travel by air. The thought of an upcoming flight can cause great distress, particularly when compelled to travel by air. The most extreme manifestations can include panic attacks or vomiting at the mere sight or mention of an aircraft or air travel.

For a long time, the fear of falling was merely believed to be a result of the psychological trauma of a fall, also called "post-fall syndrome". This syndrome was first mentioned in 1982 by Murphy and Isaacs, who noticed that after a fall, ambulatory persons developed intense fear and walking disorders. Fear of falling has been identified as one of the key symptoms of this syndrome. Since that time, FOF has gained recognition as a specific health problem among older adults. However, FOF was also commonly found among elderly persons who had not yet experienced a fall.

Prevalence of FOF appears to increase with age and to be higher in women. Age remains significant in multiple logistic regression analyses. The results of different studies have reported gender as a somewhat significant risk factor for fear of falling. Other risk factors of fear of falling in the elderly include dizziness, self-rated health status, depression, and problems with gait and balance.

Specific phobias have a one-year prevalence of 8.7% in the USA with 21.9% of the cases being severe, 30.0% moderate and 48.1% mild. The usual age of onset is childhood to adolescence. Women are twice as likely to suffer from specific phobias as men.

Evolutionary psychology argues that infants or children develop specific phobias to things that could possibly harm them, so their phobias alert them to the danger.

The most common co-occurring disorder for children with a specific phobia is another anxiety disorder. Although comorbidity is frequent for children with specific phobias, it tends to be lower than for other anxiety disorders.

Onset is typically between 7 and 9 years of age. Specific phobias can occur at any age but seem to peak between 10 and 13 years of age.

Gephyrophobia is the anxiety disorder or specific phobia characterized by the fear of bridges. As a result, sufferers of gephyrophobia may avoid routes that will take them over bridges.

Some possible explanations of gephyrophobia may be the fear of driving off the bridge, the fear of a gust of wind taking one off the bridge, a fear of the structural integrity of the bridges itself, or the fear that the bridge will collapse if they try to cross it. The fear overlaps with acrophobia, the fear of heights, as gephyrophobia tends to be exacerbated in tall bridges vs. ones close to the water/ground beneath.

Dr. Michael Liebowitz, founder of the Anxiety Disorders Clinic at the New York State Psychiatric Institute, says, "It's not an isolated phobia, but usually part of a larger constellation ... It's people who get panic attacks. You get light-headed, dizzy; your heart races. You become afraid that you'll feel trapped." It is a situational phobia.

The New York State Thruway Authority will lead gephyrophobiacs over the Tappan Zee Bridge. A driver can call the authority in advance and arrange for someone to drive the car over the bridge for them. The authority performs the service about six times a year.

The Maryland Transportation Authority offers a similar service for crossing the Chesapeake Bay Bridge.

The Mackinac Bridge Authority, which oversees the Mackinac Bridge, which connects Michigan’s Upper and Lower peninsulas, will drive one's car across its span for any needy gephyrophobiacs. Some thousand drivers take advantage of this free program each year. Leslie Ann Pluhar had her Yugo blown off that bridge. Later investigation showed the driver had stopped her car over the open steel grating on the bridge's span and that a gust of wind through the grating blew her vehicle off the bridge, although this is not supported by recorded wind speed measurements taken on and around the bridge at the time of the accident.

The term "gephyrophobia" comes from Greek γέφυρα ("gephura") meaning "bridge" and φόβος ("phobos") "fear".

Studies done by psychologists Eleanor J. Gibson and Richard D. Walk have further explained the nature of this fear. One of their more famous studies is the "visual cliff. Below is their description of the cliff:

Thirty-six infants were tested in their experiments, ranging from six to fourteen months. Gibson and Walk found that when placed on the board, 27 of the infants would crawl on the shallow side when called by their mothers; only three ventured off the "edge" of the cliff. Many infants would crawl away from their mothers who were calling from the deep end, and some would cry because they couldn’t reach their mothers without crossing an apparent chasm. Some would pat the glass on the deep end, but even with this assurance would not crawl on the glass. These results, although unable to prove that this fear is innate, indicate that most human infants have well developed depth perception and are able to make the connection between depth and the danger that accompanies falling.

Rachman proposed three pathways to acquiring fear conditioning: classical conditioning, vicarious acquisition and informational/instructional acquisition.

Much of the progress in understanding the acquisition of fear responses in phobias can be attributed to classical conditioning (Pavlovian model). When an aversive stimulus and a neutral one are paired together, for instance when an electric shock is given in a specific room, the subject can start to fear not only the shock but the room as well. In behavioral terms, this is described as a conditioned stimulus (CS) "(the room)" that is paired with an aversive unconditioned stimulus (UCS) "(the shock)", which leads to a conditioned response (CR) "(fear for the room)" (CS+UCS=CR).

For instance, in case of the fear of heights (acrophobia), the CS is heights such as a balcony on the top floors of a high rise building. The UCS originates from an aversive or traumatizing event in the person's life, such as almost falling down from a great height. The original fear of almost falling down is associated with being on a high place, leading to a fear of heights. In other words, the CS "(heights)" associated with the aversive UCS "(almost falling down)" leads to the CR "(fear)".

This direct conditioning model, though very influential in the theory of fear acquisition, is not the only way to acquire a phobia.

Vicarious fear acquisition is learning to fear something, not by a subject's own experience of fear, but by watching others reacting fearfully (observational learning). For instance, when a child sees a parent reacting fearfully to an animal, the child can become afraid of the animal as well. Through observational learning, humans are able to learn to fear potentially dangerous objects; a reaction which also been observed in non-human primates. In a study focusing on non-human primates, results showed that the primates learned to fear snakes at a fast rate after observing parents’ fearful reactions. An increase of fearful behaviors was observed as the non-human primates continued to observe their parents’ fearful reaction. Even though observational learning has been proven to be effective in creating reactions of fear and phobias, it has also been shown that by physically experiencing an event, chances increase of fearful and phobic behaviors. In some cases, physically experiencing an event may increase the fear and phobia more so than observing a fearful reaction of another human or non-human primate.

Informational/instructional fear acquisition is learning to fear something by getting information. For instance, fearing electrical wire after having heard that touching it will result in an electric shock.

A conditioned fear response to an object or situation is not always a phobia. To meet the criteria for a phobia there must also be symptoms of impairment and avoidance. Impairment is defined as being unable to complete routine tasks whether occupational, academic or social. In acrophobia an impairment of occupation could result from not taking a job solely because of its location at the top floor of a building, or socially not participating in a social event at a theme park. The avoidance aspect is defined as behavior that results in the omission of an aversive event that would otherwise occur with the goal of the preventing anxiety.

A specific phobia is any kind of anxiety disorder that amounts to an unreasonable or irrational fear related to exposure to specific objects or situations. As a result, the affected person tends to avoid contact with the objects or situations and, in severe cases, any mention or depiction of them. The fear can, in fact, be disabling to their daily lives.

The fear or anxiety may be triggered both by the presence and the anticipation of the specific object or situation. A person who encounters that of which they are phobic will often show signs of fear or express discomfort. In some cases, it can result in a panic attack. In most adults, the person may logically know the fear is unreasonable but still find it difficult to control the anxiety. Thus, this condition may significantly impair the person's functioning and even physical health.

Specific phobia affects up to 12% of people at some point in their life.

Gephyrophobia is the name of a map in the video game ""; the two bases are connected only by a bridge flanked by snipers' outposts. The name most likely comes from the fact that the players are frequently killed on the bridge.