The Decline of the West is a two-volume work by Oswald Spengler. The first volume, subtitled Form and Actuality, was published in the summer of 1918. The second volume, subtitled Perspectives of World History, was published in 1922. The definitive edition of both volumes was published in 1923.

Oswald Arnold Gottfried Spengler was a German polymath, whose areas of interest included history, philosophy, mathematics, science, and art, as well as their relation to his organic theory of history. He is best known for his two-volume work The Decline of the West, published in 1918 and 1922, covering human history. Spengler's model of history postulates that human cultures and civilizations are akin to biological entities, each with a limited, predictable, and deterministic lifespan.