Aleister Crowley (/ˈælɪstər ˈkroʊli/ AL-ist-ər KROH-lee; born Edward Alexander Crowley; 12 October 1875 – 1 December 1947) was an English occultist, ceremonial magician, poet, novelist, mountaineer, and painter. He founded the religion of Thelema, identifying himself as the prophet entrusted with guiding humanity into the Æon of Horus in the early 20th century. As a prolific writer, he published widely over the course of his life.

Born to a wealthy family in Royal Leamington Spa, Warwickshire, Crowley rejected his parents' fundamentalist Christian Plymouth Brethren faith to pursue an interest in Western esotericism. He was educated at Trinity College at the University of Cambridge, where he focused his attention upon mountaineering and poetry, resulting in several publications. Some biographers allege that here he was recruited into a British intelligence agency, further suggesting that he remained a spy throughout his life. In 1898, he joined the esoteric Hermetic Order of the Golden Dawn, where he was trained in ceremonial magic by Samuel Liddell MacGregor Mathers and Allan Bennett. He went mountaineering in Mexico with Oscar Eckenstein, before studying Hindu and Buddhist practices in India. In 1904, he married Rose Edith Kelly, and they honeymooned in Cairo, Egypt, where Crowley wrote down The Book of the Law—a sacred text that serves as the basis for Thelema, which he said had been dictated to him by a supernatural entity named Aiwass. The Book announced the start of the Æon of Horus and declared that its followers should "Do what thou wilt": seek to align themselves with their True Will via the practice of ceremonial magic.

After the unsuccessful 1905 Kanchenjunga expedition and a visit to India and China, Crowley returned to Britain, where he attracted attention as a prolific author of poetry, novels, and occult literature. In 1907, he and George Cecil Jones co-founded an esoteric order—the A∴A∴, through which they propagated Thelema. After spending time in Algeria, in 1912 he was initiated into another esoteric order, the German-based Ordo Templi Orientis (O.T.O.); he rose to become the leader of its British branch, which he reformulated in accordance with Thelema. Through O.T.O., Thelemite groups were established in Britain, Australia, and North America. Crowley spent the First World War in the United States, where he took up painting, and campaigned for the German war effort against Britain. His biographers later revealed that he had infiltrated the pro-German movement to assist the British intelligence services. In 1920, he established the Abbey of Thelema, a religious commune in Cefalù, Sicily, where he lived with various followers. His libertine lifestyle led to denunciations in the British press, and the Italian government evicted him in 1923. He divided the following two decades between France, Germany, and England, and continued to promote Thelema until his death.

Crowley gained widespread notoriety during his lifetime, being a drug user, a bisexual, and an individualist social critic. Crowley has remained a highly influential figure over western esotericism and the counterculture of the 1960s, and he continues to be considered a prophet in Thelema. He is the subject of various biographies and academic studies.

Crowley was born Edward Alexander Crowley at 30 Clarendon Square in Royal Leamington Spa, Warwickshire, England, on 12 October 1875. His father, Edward Crowley (1829–1887), was trained as an engineer, but his share in a lucrative family brewing business, Crowley's Alton Ales, allowed him to retire before his son was born. His mother, Emily Bertha Bishop (1848–1917), came from a Devonshire-Somerset family and had a strained relationship with her son; she described him as "the Beast", a name that he revelled in. The couple had been married at London's Kensington Registry Office in November 1874, and were evangelical Christians. Crowley's father was born a Quaker, but converted to the Exclusive Brethren, a faction of a Christian fundamentalist group known as the Plymouth Brethren; Emily likewise converted upon marriage. Crowley's father was particularly devout, spending his time as a travelling preacher for the sect and reading a chapter from the Bible to his wife and son after breakfast every day. Following the death of their baby daughter in 1880, in 1881 the Crowleys moved to Redhill, Surrey. At the age of 8, Crowley was sent to H. T. Habershon's evangelical Christian boarding school in Hastings, and then to Ebor preparatory school in Cambridge, run by the Reverend Henry d'Arcy Champney, whom Crowley considered a sadist.

In March 1887, when Crowley was eleven years old, his father died of tongue cancer. Crowley described this as a turning point in his life, and he always maintained an admiration of his father, describing him as "my hero and my friend". Inheriting a third of his father's wealth, he began misbehaving at school and was harshly punished by Champney; Crowley's family removed him from the school when he developed albuminuria. He then attended Malvern College and Tonbridge School, both of which he despised and left after a few terms. He became increasingly sceptical of Christianity, pointing out Biblical inconsistencies to his religious teachers, and went against the Christian morality of his upbringing by smoking, masturbating, and having sex with prostitutes from whom he contracted gonorrhea. Sent to live with a Brethren tutor in Eastbourne, he undertook chemistry courses at Eastbourne College. Crowley developed interests in chess, poetry, and mountain climbing, and in 1894 climbed Beachy Head before visiting the Alps and joining the Scottish Mountaineering Club. The following year he returned to the Bernese Alps, climbing the Eiger, Trift, Jungfrau, Mönch, and Wetterhorn.

Having adopted the name of Aleister over Edward, in October 1895 Crowley began a three-year course at Trinity College, Cambridge, where he was entered for the Moral Science Tripos studying philosophy. With approval from his personal tutor, he changed to English literature, which was not then part of the curriculum offered. Crowley spent much of his time at university engaged in his pastimes, becoming president of the chess club and practising the game for two hours a day; he briefly considered a professional career as a chess player. Crowley also embraced his love of literature and poetry, particularly the works of Richard Francis Burton and Percy Bysshe Shelley. Many of his own poems appeared in student publications such as The Granta, Cambridge Magazine, and Cantab. He continued his mountaineering, going on holiday to the Alps to climb every year from 1894 to 1898, often with his friend Oscar Eckenstein, and in 1897 he made the first ascent of the Mönch without a guide. These feats led to his recognition in the Alpine mountaineering community.

For many years I had loathed being called Alick, partly because of the unpleasant sound and sight of the word, partly because it was the name by which my mother called me. Edward did not seem to suit me and the diminutives Ted or Ned were even less appropriate. Alexander was too long and Sandy suggested tow hair and freckles. I had read in some book or other that the most favourable name for becoming famous was one consisting of a dactyl followed by a spondee, as at the end of a hexameter: like Jeremy Taylor. Aleister Crowley fulfilled these conditions and Aleister is the Gaelic form of Alexander. To adopt it would satisfy my romantic ideals.