Dame Agatha Christie DBE (15 September 1890 – 12 January 1976), was an English crime writer of novels, short stories and plays. She also wrote romances under the name Mary Westmacott, but is best remembered for her 80 detective novels and her successful West End theatre plays. Her works, particularly those featuring detectives Hercule Poirot and Miss Jane Marple, have given her the title the 'Queen of Crime' and made her one of the most important and innovative writers in the development of the genre.

Christie has been referred to by the Guinness Book of World Records as the best-selling writer of books of all time and the best-selling writer of any kind, along with William Shakespeare. Only the Bible is known to have outsold her collected sales of roughly four billion copies of novels. UNESCO states that she is currently the most translated individual author in the world with only the collective corporate works of Walt Disney Productions surpassing her. Christie's books have been translated into at least 56 languages. Her stage play The Mousetrap holds the record for the longest initial run in the world: it opened at the Ambassadors Theatre in London on 25 November 1952 and as of 2010 is still running after more than 23,000 performances. In 1955, Christie was the first recipient of the Mystery Writers of America's highest honour, the Grand Master Award, and in the same year, Witness for the Prosecution was given an Edgar Award by the MWA, for Best Play. Most of her books and short stories have been filmed, some many times over (Murder on the Orient Express, Death on the Nile and 4.50 From Paddington for instance), and many have been adapted for television, radio, video games and comics. In 1968, Booker Books, a subsidiary of the agri-industrial conglomerate Booker-McConnell, bought a 51 percent stake in Agatha Christie Limited, the private company that Christie had set up for tax purposes. Booker later increased its stake to 64 percent. In 1998, Booker sold its shares to Chorion, a company whose portfolio also includes the literary estates of Enid Blyton and Dennis Wheatley. In 2004, a 5,000-word story entitled "The Incident of the Dog's Ball" was found in the attic of the author's daughter. It was published in Britain in September 2009. On November 10, 2009, Reuters announced that the story will be published by The Strand Magazine.

Agatha Mary Clarissa Miller was born in Torquay, Devon, England. Her mother, Clarissa Margaret Boehmer, was the daughter of a British army captain, but had been sent, as a child, to live with her own mother's sister, who was the second wife of a wealthy American. Eventually Margaret married her stepfather's son from his first marriage, Frederick Alvah Miller, an American stockbroker. Thus the two women Agatha called "Grannie" were sisters. Despite her father's nationality as a "New Yorker" and her aunt's relation to the Pierpont Morgans, Agatha never claimed United States citizenship or connection. The Millers had two other children: Margaret Frary Miller (1879–1950), called Madge, who was eleven years Agatha's senior, and Louis Montant Miller (1880–1929), called Monty, ten years older than Agatha. Later, in her autobiography, Agatha would refer to her brother as "an amiable scapegrace of a brother".

During the First World War, she worked at a hospital as a nurse; she liked the profession, calling it "one of the most rewarding professions that anyone can follow". She later worked at a hospital pharmacy, a job that influenced her work, as many of the murders in her books are carried out with poison. Despite a turbulent courtship, on Christmas Eve 1914 Agatha married Archibald Christie, an aviator in the Royal Flying Corps. The couple had one daughter, Rosalind Hicks. They divorced in 1928, two years after Christie discovered her husband was having an affair. It was during this marriage that she published her first novel in 1920, The Mysterious Affair at Styles. In 1924, she published a collection of mystery and ghost stories entitled The Golden Ball. In late 1926, Agatha's husband Archie revealed that he was in love with another woman, Nancy Neele, and wanted a divorce. On 8 December 1926, the couple quarrelled, and Archie Christie left their house in Sunningdale, Berkshire, to spend the weekend with his mistress at Godalming, Surrey. That same evening Agatha disappeared from her home, leaving behind a letter for her secretary saying that she was going to Yorkshire. Her disappearance caused an outcry from the public, many of whom were admirers of Agatha Christie's novels. Despite a massive manhunt, there were no results until eleven days later. Eleven days after her disappearance, Christie was identified as a guest at the Swan Hydropathic Hotel (now the Old Swan Hotel) in Harrogate, Yorkshire where she was registered as 'Mrs Teresa Neele' from Cape Town. Christie gave no account of her disappearance. Although two doctors had diagnosed her as suffering from amnesia, opinion remains divided as to the reasons for her disappearance. One suggestion is that she had suffered a nervous breakdown brought about by a natural propensity for depression, exacerbated by her mother's death earlier that year, and the discovery of her husband's infidelity. Public reaction at the time was largely negative with many believing it was all just a publicity stunt, whilst others speculated she was trying to make the police think her husband killed her as revenge for his affair.

In 1930, Christie married archaeologist Max Mallowan after joining him in an archaeological dig. Their marriage was especially happy in the early years and remained so until Christie's death in 1976. In 1977, Mallowan married his longtime associate, Barbara Parker. Christie's travels with Mallowan contributed background to several of her novels set in the Middle East. Other novels (such as And Then There Were None) were set in and around Torquay, where she was born. Christie's 1934 novel, Murder on the Orient Express was written in the Hotel Pera Palace in Istanbul, Turkey, the southern terminus of the railway. The hotel maintains Christie's room as a memorial to the author. The Greenway Estate in Devon, acquired by the couple as a summer residence in 1938, is now in the care of the National Trust. Christie often stayed at Abney Hall in Cheshire, which was owned by her brother-in-law, James Watts. She based at least two of her stories on the hall: The short story The Adventure of the Christmas Pudding, which is in the story collection of the same name, and the novel After the Funeral. "Abney became Agatha's greatest inspiration for country-house life, with all the servants and grandeur which have been woven into her plots. The descriptions of the fictional Styles, Chimneys, Stoneygates and the other houses in her stories are mostly Abney in various forms."

During the Second World War, Christie worked in the pharmacy at University College Hospital of University College, London, where she acquired a knowledge of poisons that she put to good use in her post-war crime novels. For example, the use of thallium as a poison was suggested to her by UCH Chief Pharmacist Harold Davis (later appointed Chief Pharmacist at the UK Ministry of Health), and in The Pale Horse, published in 1961, she employed it to dispatch a series of victims, the first clue to the murder method coming from the victims’ loss of hair. So accurate was her description of thallium poisoning that on at least one occasion it helped solve a case that was baffling doctors.

To honour her many literary works, she was appointed Commander of the Order of the British Empire in the 1956 New Year Honours. The next year, she became the President of the Detection Club. In the 1971 New Year Honours she was promoted Dame Commander of the Order of the British Empire, three years after her husband had been knighted for his archeological work in 1968. They were one of the few married couples where both partners were honoured in their own right.

From 1971 to 1974, Christie's health began to fail; however, she continued to write. Recently, using experimental, computerized, textual tools of analysis, Canadian researchers have suggested that Christie may have begun to suffer from Alzheimer's disease or other dementia. In 1975, sensing her increasing weakness, Christie signed over the rights of her most successful play, The Mousetrap, to her grandson. Agatha Christie died on 12 January 1976, at age 85, from natural causes, at her Winterbrook House in the north of Cholsey parish, adjoining Wallingford in Oxfordshire (formerly Berkshire). She is buried in the nearby churchyard of St Mary's, Cholsey.

Christie's only child, Rosalind Margaret Hicks died, also aged 85, on 28 October 2004 from natural causes, in Torbay, Devon. Christie's grandson, Mathew Prichard, was heir to the copyright to some of his grandmother's literary work (including The Mousetrap) and is still associated with Agatha Christie Limited.

Agatha Christie's first novel The Mysterious Affair at Styles was published in 1920 and introduced the long-running character detective Hercule Poirot, who appeared in 33 of Christie's novels and 54 short stories. Her other well known character, Miss Marple, was introduced in The Tuesday Night Club in 1927 (short story), and was based on women like Christie's grandmother and her "cronies". During the Second World War, Christie wrote two novels, Curtain and Sleeping Murder, intended as the last cases of these two great detectives, Hercule Poirot and Jane Marple, respectively. Both books were sealed in a bank vault for over thirty years, and were released for publication by Christie only at the end of her life, when she realized that she could not write any more novels. These publications came on the heels of the success of the film version of Murder on the Orient Express in 1974. Like Arthur Conan Doyle with Sherlock Holmes, Christie was to become increasingly tired of her detective, Poirot. In fact, by the end of the 1930s, Christie confided to her diary that she was finding Poirot “insufferable," and by the 1960s she felt that he was "an ego-centric creep." However, unlike Conan Doyle, Christie resisted the temptation to kill her detective off while he was still popular. She saw herself as an entertainer whose job was to produce what the public liked, and the public liked Poirot. In contrast, Christie was fond of Miss Marple. However, it is interesting to note that the Belgian detective’s titles outnumber the Marple titles by more than two to one. This is largely because Christie wrote numerous Poirot novels early in her career, while The Murder at the Vicarage remained the sole Marple novel until the 1940s. Christie never wrote a novel or short story featuring both Poirot and Miss Marple. In a recording, recently re-discovered and released in 2008, Christie revealed the reason for this: "Hercule Poirot, a complete egoist, would not like being taught his business or having suggestions made to him by an elderly spinster lady". Poirot is the only fictional character to have been given an obituary in The New York Times, following the publication of Curtain in 1975. Following the great success of Curtain, Christie gave permission for the release of Sleeping Murder sometime in 1976, but died in January 1976 before the book could be released. This may explain some of the inconsistencies compared to the rest of the Marple series — for example, Colonel Arthur Bantry, husband of Miss Marple's friend, Dolly, is still alive and well in Sleeping Murder despite the fact he is noted as having died in books published earlier. It may be that Christie simply did not have time to revise the manuscript before she died. Miss Marple fared better than Poirot, since after solving the mystery in Sleeping Murder she returns home to her regular life in St. Mary Mead.

On an edition of Desert Island Discs in 2007, Brian Aldiss claimed that Agatha Christie told him that she wrote her books up to the last chapter, and then decided who the most unlikely suspect was. She would then go back and make the necessary changes to "frame" that person. The evidence of Christie's working methods, as described by successive biographers, contradicts this claim. Almost all of Agatha Christie’s books are whodunits, focusing on the English middle and upper classes. Usually, the detective either stumbles across the murder or is called upon by an old acquaintance, who is somehow involved. Gradually, the detective interrogates each suspect, examines the scene of the crime and makes a note of each clue, so readers can analyze it and be allowed a fair chance of solving the mystery themselves. Then, about halfway through, or sometimes even during the final act, one of the suspects usually dies, often because they have inadvertently deduced the killer's identity and need silencing. In a few of her novels, including Death Comes as the End and And Then There Were None, there are multiple victims. Finally, the detective organizes a meeting of all the suspects and slowly denounces the guilty party, exposing several unrelated secrets along the way, sometimes over the course of thirty or so pages. The murders are often extremely ingenious, involving some convoluted piece of deception. Christie’s stories are also known for their taut atmosphere and strong psychological suspense, developed from the deliberately slow pace of her prose. Twice, the murderer surprisingly turns out to be the unreliable narrator of the story. In four stories, Christie allows the murderer to escape justice (and in the case of the last three, implicitly almost approves of their crimes); these are The Witness for the Prosecution, Murder on the Orient Express, Curtain and The Unexpected Guest. After the dénouement of Taken at the Flood, her sleuth Poirot has the guilty party arrested for the lesser crime of manslaughter. (When Christie adapted Witness into a stage play, she lengthened the ending so that the murderer was also killed.) There are also numerous instances where the killer is not brought to justice in the legal sense but instead dies (death usually being presented as a more 'sympathetic' outcome), for example Death on the Nile, The Murder of Roger Ackroyd, Crooked House, Appointment with Death and The Hollow. In some cases this is with the collusion of the detective involved. Five Little Pigs, and arguably Ordeal by Innocence, end with the question of whether formal justice will be done unresolved.

Agatha Christie was revered as a master of suspense, plotting, and characterization by most of her contemporaries and, even today, her stories have received glowing reviews in most literary circles. Fellow crime writer Anthony Berkeley Cox was an admitted fan of her work, once saying that nobody can write an Agatha Christie novel but the authoress herself. However, she does have her detractors, most notably the American novelist Raymond Chandler, who criticised her in his essay, The Simple Art of Murder, and the American literary critic Edmund Wilson, who was dismissive of Christie and the detective fiction genre generally in his New Yorker essay, "Who Cares Who Killed Roger Ackroyd?". Christie occasionally inserted stereotyped descriptions of characters into her work, particularly before the end of the Second World War (when such attitudes were more commonly expressed publicly), and particularly in regard to Italians, Jews, and non-Europeans generally. For example, in the first editions of the collection The Mysterious Mr Quin (1930), in the short story "The Soul of the Croupier," she described "Hebraic men with hook-noses wearing rather flamboyant jewellery"; in later editions the passage was edited to describe "sallow men" wearing same.