O'Neill, Eugene Gladstone (1888-1953), American dramatist, Nobel laureate, winner of four Pulitzer Prizes, who attempted to define fundamental human problems in his works. He is considered by many to be the most important writer in the American theater.

O'Neill was born in New York City, the son of the Irish-American actor James O'Neill. He accompanied his father on theatrical tours during his youth, attended Princeton University from 1906 to 1907, and worked subsequently as a clerk in New York City. From 1909 to 1912 O'Neill prospected for gold in Honduras, served as assistant manager of a theatrical troupe organized by his father, went to South America and South Africa as a seaman, toured as an actor with his father's troupe, and worked as a newspaper reporter in New London, Connecticut. After contracting a mild case of tuberculosis in 1912, he went to a sanatorium, where he wrote his first plays. After leaving the sanatorium, O'Neill studied the techniques of playwriting at Harvard University from 1914 to 1915 under the famous theater scholar George Pierce Baker.

During most of the next ten years O'Neill lived in Provincetown, Massachusetts, and in New York City, where he served as both a dramatist and a manager for the Provincetown Players. This experimental theatrical group staged a number of his one-act plays, beginning with Bound East for Cardiff (1916), and several long plays, including The Hairy Ape (1922). Beyond the Horizon (1920; Pulitzer Prize, 1921), a domestic tragedy in three acts, was produced successfully on the Broadway stage, as was The Emperor Jones (1920), a study of the disintegration of the mind of a black dictator under the influence of fear. In the nine-act play Strange Interlude (1928; Pulitzer Prize) O'Neill sought to portray the way in which hidden psychological processes impinge upon outward actions. His most ambitious work, the trilogy Mourning Becomes Electra (1931), was an attempt to re-create the power and profundity of the ancient Greek tragedies by setting the themes and plot of the Oresteia by Aeschylus in 19th-century New England. Ah, Wilderness (1933), written in a relatively light vein, was another of his most successful plays.

O'Neill's other dramas include Moon of the Caribbees (1918), Anna Christie (1921; Pulitzer Prize, 1922), All God's Chillun Got Wings (1924), Desire Under the Elms (1924), The Great God Brown (1926), Lazarus Laughed (1928), Marco Millions (1928), Dynamo (1929), and Days Without End (1934).

From 1934 until his death O'Neill suffered from a crippling nervous disorder similar to Parkinson's disease. During this entire period he worked intermittently on a long cycle of plays concerning the history of an American family, but he completed only A Touch of the Poet (produced posthumously 1957) and More Stately Mansions (produced posthumously in Sweden 1962; produced in the United States 1967). After 1939 he wrote three other plays unrelated to the cycle: The Iceman Cometh (1946), which portrays a group of social misfits unable to live without illusions, and two tragedies dealing with his family, Long Day's Journey into Night (produced posthumously 1956; Pulitzer Prize, 1957) and A Moon for the Misbegotten (produced posthumously 1957). O'Neill was awarded the 1936 Nobel Prize for literature.

Many of O'Neill's dramas are marked by new theatrical techniques and symbolic devices that express religious and philosophical ideas and give his characters psychological depth. He employed the sound of tom-toms gradually increasing in volume to mark an increase in tension, masks to indicate shadings of personality, lengthy asides to allow his characters to voice their thoughts, and choruses used as in ancient Greek tragedies to comment on the play's action.

O'Neill's best works convey forcibly his vision of modern humans as victims of circumstances who cannot believe in God, destiny, or free will and who therefore blame impersonal causes for their misery and punish themselves for their own sin and guilt. Despite the seriousness and theatrical brilliance of many of O'Neill's plays, much of his symbolism is obscure, and his innovations in stagecraft often do not achieve the desired effects. In addition, the language of his characters has been criticized for lapses into banality or bathos at many of the most compelling moments of his plays. By bringing psychological realism, philosophical depth, and poetic symbolism into the American theater, however, O'Neill's work raised the standards of most later American dramatists.