Therapeutic interventions involving the use of living animals have a surprisingly long and distinguished pedigree. Dogs, for example, played a central role in the ancient Greek cult of Asklepios (Aesculapius), the son of Apollo, who was known as the God of Medicine and the Divine Physician. Asklepios's shrine in the sacred grove at Epidaurus functioned as a kind of ancient health resort where crowds of suppliants sought relief from a great variety of maladies. Treatment involved various rites of purification and sacrifice followed by periods of drug-induced sleep within the main body of the shrine during which the God visited each of his 'patients', sometimes in human form but more often in the guise of a dog that licked them on the relevant injured or ailing portions of their anatomy. It appears that the dogs that lived around the shrine were trained to lick people, and that it was believed that these animals had the power to cure illness with their tongues.

The notion that dogs could heal injuries or sores by touching or licking them also persisted into the Middle Ages. St Roch who, like Asklepios, was generally depicted in the company of a dog, was supposedly cured of plague sores by the licking of his canine companion. St Christopher, St Bernard and a number of other saints were also associated with dogs, and many of them had reputations as healers.

The notion that nurturing relationships with animals could serve a socialising function, especially for children, first surfaced during the early modern period. Writing in 1699, John Locke advocated giving children 'dogs, squirrels, birds or any such things' to look after as a means of encouraging them to develop tender feelings and a sense of responsibility for others. Deriving their authority from the works of John Calvin and Thomas Hobbes, many eighteenth-century reformers believed that children could learn to reflect on, and control, their own innately beast-like characteristics through the act of caring for and controlling real animals.

In the late eighteenth century, theories concerning the socialising influence of animal companionship also began to be applied to the treatment of the mentally ill. The earliest well documented experiment of this kind took place in England at The York Retreat, the brainchild of a progressive Quaker called William Tuke. The York Retreat employed treatment methods which were exceptionally enlightened when compared with those which existed in other mental institutions of the day. Inmates were permitted to wear their own clothing, were encouraged to engage in handicrafts, and were allowed to wander freely around the retreat's courtyards and gardens which contained various small domestic animals. In his Description of the Retreat (1813), Samuel Tuke, the founder's grandson, described how the internal courtyards of the Retreat were supplied 'with a number of animals; such as rabbits, sea-gulls, hawks, and poultry. These creatures are generally very familiar with the patients: and it is believed they are not only the means of innocent pleasure; but that the intercourse with them, sometimes tends to awaken the social and benevolent feelings.'

During the nineteenth century, pet animals became increasingly common features of mental institutions in England and elsewhere. In a highly critical report on the appalling conditions endured by the inmates of Bethlem Hospital during the 1830s, the British Charity Commissioners suggested that the grounds of lunatic asylums 'should be stocked with sheep, hares, a monkey, or some other domestic or social animals' to create a more pleasing and less prison-like atmosphere. Such recommendations were evidently taken seriously. According to an article published in the Illustrated London News of 1860, the women's ward at the Bethlem Hospital was by that time 'cheerfully lighted, and enlivened with prints and busts, with aviaries and pet animals', while in the men's ward the same fondness was manifested 'for pet birds and animals, cats, canaries, squirrels, greyhounds &... [some patients] pace the long gallery incessantly, pouring out their woes to those who listen to them, or, if there be none to listen, to the dogs and cats...'.

The beneficial effects of animal companionship also appear to have been recognised as serving a therapeutic role in the treatment of physical ailments during this period. In her Notes on Nursing (1880), for instance, Florence Nightingale observed that a small pet 'is often an excellent companion for the sick, for long chronic cases especially.'

The revival of interest in the therapeutic benefits of animal-assisted interventions during the last 30–40 years can be attributed largely to the writings of the American child psychologist, Boris Levinson. Although Levinson speculated at considerable length on the potential beneficial influence of pets on human development, his chief contribution was the entirely novel idea of using pets as co-therapists during psychotherapeutic counselling. Levinson observed that many of his more withdrawn patients readily related to, and interacted with, his pet dog when they were too withdrawn to interact with him. He found that by carefully involving himself in this animal-child relationship he was able to break down the child's initial hostility and reserve and establish therapeutic rapport far more rapidly than was otherwise possible. Pets not only served as 'ice-breakers' in this context, but also seemed to provide the child with a relatively neutral medium through which to express unconscious emotional conflicts, worries and fears.

Perhaps surprisingly, Levinson did not endorse the popular idea that all children instinctively identify more easily with animals than with adult humans. Instead, he favoured the view that emotionally disturbed children who have experienced difficulties in their relationships with adults relate more easily or quickly to animals. Thus, for Levinson, the relationship between the disturbed child and the pet represented a kind of emotional bridge which, if carefully exploited, could be used to reawaken the child's enthusiasm for interpersonal relationships.

Although Boris Levinson's early publications on 'pet-facilitated psychotherapy' received a distinctly lukewarm response from his peers, many of his original ideas have been substantially vindicated by the results of subsequent research.