Aleister Crowley was a mystic and author who was nicknamed The Wickedest Man in the World'. He was also notorious for the number of pseudonyms he used for the many books and articles he wrote.

One estimate is that he used at least 150 pen-names, including Mrs Bloomer Greymare, Lemuel S. Innocent, A Gentleman of the University of Cambridge, Comte de Fenix, Ethel Ramsay, Lord Boleskine, Mahatma Guru Sri Paramahansa Shivaji, Hilda Norfolk, and Percy Flage.

In fact his most familiar name - Aleister Crowley - was partly a pseudonym, since he had been christened Edward Alexander Crowley and was nicknamed Aleck by his mother, which he didn't like. He chose his new name because he had read somewhere that "the most favourable name for becoming famous was one consisting of a dactyl followed by a spondee - like Jeremy Taylor'."

This phenomenon of multiple pseudonyms is not as rare as you might expect, although few people match the excesses of Aleister Crowley. But Daniel Defoe was just one of nearly 200 pseudonyms used by Daniel Foe, including Nicholas Boggle and Sir Fopling Tittle-Tattle. Georges Simenon published his first novel in 1921 as G. Sim, and he subsequently used more than 20 different pen-names.

Writers of thrillers and whodunnits seem to enjoy using pseudonyms. The poet C. Day Lewis wrote detective stories under the name of Nicholas Blake; John Creasey used a variety of pseudonyms when writing his crime novels; and Ellery Queen was the pen-name chosen by Manfred B. Lee and Frederic Dannay when they collaborated on detective stories. Ian Rankin has written as Jack Harvey, and Ruth Rendell is also known as Barbara Vine.

The practice also seems to be widespread among science-fiction writers. Ray Bradbury hid behind at least 16 pseudonyms, including Omega, Doug Rogers and Cecil Claybourne Cunningham. H.P. Lovecraft used such names as Edgar Softly, Henry Paget-Lowe and Humphrey Littlewit, Esq.

Pseudonyms are often used by prolific writers to hide the fact that they are writing lots of books, or when they are writing for different publishers or magazines. Because he writes new books so frequently, Harry Patterson writes under the names of Jack Higgins, Hugh Marlow, James Graham and Martin Fallon.

When Charles Dickens published his first article in 1833, he called himself Boz, which was actually the nickname of his younger brother. Thackeray used a variety of pen names, including George Savage Fitz-Boodle, Michael Angelo Titmarsh and Charles James Yellowplush.

According to his sister, H H Munro adopted the pen name of Saki from the name of the cup-bearer in The Rubaiyat of Omar Khayyim, one of his favourite books.

Pen-names can be used to avoid persecution or censorship, as when François Marie Arouet adopted the name Voltaire so he could publish pamphlets criticising the authorities and expounding unpopular philosophies. In fact Voltaire is said to have used 173 different pseudonyms. Vladimir Ilyich Ulyanov (above) adopted the name of Lenin when he was involved in revolutionary activities.

In the 19th century, many women concealed their identities with pen-names, because serious writing was thought an unsuitable pastime for the gentle sex. Thus the Brontë sisters - Charlotte, Emily and Anne - published their novels under the names of Currer Bell, Ellis Bell and Acton Bell.

The same motive drove Mary Ann Evans to become George Eliot, and Amandine Dudevant to call herself George Sand. When Louisa M. Alcott (of Little Women fame) published her Hospital Sketches in 1863, she used the pen-name Tribulation Periwinkle.

Even later in the 19th century, the South African writer Olive Schreiner called herself Ralph Iron when she issued her first novel The Story of an African Farm in 1883, because her campaigning feminist views were likely to be unpopular.

The writer Winifred Ashton took her pen-name, Clemence Dane, from St Clement Danes church in London. Eric Blair employed a place-name in his pseudonym, George Orwell - constructed from the names of the patron saint of England and a river in Suffolk that he loved.

Actors and entertainers often resort to pseudonyms. To join Equity, the British actors' trade union, you cannot have the same name as someone else in the business. So the British actor James Stewart changed his name to Stewart Granger to avoid confusion with the American star of It's a Wonderful Life. And David McDonald couldn't use his real name when he joined Equity, so he became David Tennant, the current Dr Who.

British ballet-dancers used to adopt Russian-sounding names to give the impression that they might have trained with the Bolshoi. Hilda Munnings became Lydia Sokolova; Hilda Boot called herself Hilda Butsova; Anton Dolin was the pseudonym of Patrick Healey-Kay, who borrowed the name Anton from Anton Chekhov. Diaghilev changed Lilian Alice Marks to Alicia Markova, but he anglicised Gyorgi Balanchivadze to George Balanchine. Margot Fonteyn was originally Margaret Hookham: her adopted surname came from her Brazilian mother's maiden name, Fontes.

ony Augarde is the author of The Oxford Guide to Word Games (OUP, £14.99) and The Oxford A to Z of Word Games (OUP, £4.99).