Long Day's Journey into Night is a drama play in four acts written by American playwright Eugene O'Neill in 1941–42 but first published in 1956. The play is widely considered to be his magnum opus and one of the finest American plays of the 20th century. It premiered in Sweden in February 1956 and then opened on Broadway in November 1956, winning the Tony Award for Best Play.
O'Neill posthumously received the 1957 Pulitzer Prize for Drama for Long Day's Journey into Night. The work concerns the Tyrone family – consisting of parents James and Mary and their sons Edmund and Jamie. Mary is addicted to morphine and Edmund is ill with tuberculosis. The "Long Day" refers to the setting of the play, which takes place during one day. The play is semi-autobiographical.