Eugene Gladstone O'Neill (October 16, 1888 – November 27, 1953) was an American playwright. His poetically titled plays were among the first to introduce into the U.S. the drama techniques of realism, earlier associated with Russian playwright Anton Chekhov, Norwegian playwright Henrik Ibsen, and Swedish playwright August Strindberg. The tragedy Long Day's Journey into Night is often included on lists of the finest U.S. plays in the 20th century, alongside Tennessee Williams's A Streetcar Named Desire and Arthur Miller's Death of a Salesman. He was awarded the 1936 Nobel Prize in Literature.

O'Neill's plays were among the first to include speeches in American English vernacular and involve characters on the fringes of society. They struggle to maintain their hopes and aspirations, but ultimately slide into disillusion and despair. Of his very few comedies, only one is well-known (Ah, Wilderness!). Nearly all of his other plays involve some degree of tragedy and personal pessimism.