Long Day's Journey into Night was written in 1940 but not staged until 1956, after O'Neill's death. Unashamedly autobiographical, it is, as he puts it himself in the dedicatory note, 'a play of old sorrow, written in tears and blood', a harrowing attempt to understand himself and his family.

This is as close as the public will come to an autobiography, although its presentation in play form will perhaps mislead some potential readers. Written in 1940, now posthumously published, this autobiographical play shows the famous playwright grappling with the raw material of his youth and home surroundings. It is starkly realistic, gloomy to the point of desperation, often dull and formless - but it is a brave attempt to face the implications of a difficult heritage. These were indeed discouraging:- a swashbuckling Irish father, who had attained a certain prosperity as a handsome actor, who was given to alternate ??uts of drunken br??g??doce to and sentimentality; an older brother, of the ne'er de well type, given to drink, whoring and sloth, who showed "Edmund" (the consumptive protagonist of the play- presumably O'Neill himself) a baffling, Janus-faced love and jealousy; and a mother, worn out both by conflict with these male characters and a homeless; up-rooted life, who becomes a hopeless drug addict. It is impossible to say that O'Neill has fashioned a real play out of the Ibaenc??que facts. He has merely told his story in conversational form, his accustomed medium, devoid- this time of his poetic language. Certainly this is not stage material. Let us hope the little theatres will not claim it as their own. Rather it is material for biographers and psychologists, particularly those seeking clues to genius. For a selected audience, and definitely not for bed- time reading. (Kirkus Reviews)