Otto Penzler in his book, 101 Greatest Films of Mystery & Suspense (ibooks, NY, 2000):
If any one film could be said to epitomize the term film noir, Out of the Past would be it. The tough hero who is doomed for love of the wrong woman; the treacherous femme fatale who double-crosses every man she meets; the inevitability of the past resurfacing to assure violent death; the night, when everything seems to happen so commonly that daylight seems an intrusion; gangsters; nightclubs; jazz; bright lights; deep shadows; a good woman lost; dialogue that sounds like pulp poetry – all of it and more can be found in Out of the Past. (p.281)
Penzler also quotes what must be one of the great film noir lines, spoken by the hero to the femme-fatale: “You’re like a leaf that the wind blows from one gutter to another.”