Naturalism applies both to scientific ideas and principles, such as instinct and Darwin's theory of evolution, and to fiction. Authors in this movement wrote stories in which the characters behave in accordance with the impulses and drives of animals in nature. The tone is generally objective and distant, like that of a botanist or biologist taking notes or preparing a treatise. Naturalist writers believe that truth is found in nature, and because nature operates within consistent principles, patterns, and rules, truth is consistent.

Because the focus of Naturalism is human nature, stories in this movement are character-driven rather than plot-driven. Although Naturalism was inspired by the work of the French writer Émile Zola, it reached the peak of its accomplishment in the United States. In France, Naturalism was strongest in the late 1870s and early 1880s, but it emerged in the United States at the end of the nineteenth century and extended up to the first world war.

The fundamental naturalist doctrine is presented in Zola's 1880 essay "Le roman experimental" (meaning the experimental—or experiential— novel). In it, Zola claims that the naturalist writer should subject believable characters and events to experimental conditions. In other words, take the known (such as a character) and introduce it into the unknown (such as an unfamiliar place). Another major principle of Naturalism that Zola explains in this essay is the idea of determinism, which is the theory that a person's fate is determined solely by heredity and environment.

While the French initiated and began to develop Naturalism, Americans are credited with bringing it to fruition. American Naturalist writers include the novelists Theodore Dreiser, Stephen Crane, Frank Norris, Hamlin Garland, and Jack London; the short story writer O. Henry (William Sydney Porter); and the poets Edwin Arlington Robinson and Edgar Lee Masters. Dreiser's An American Tragedy is considered the pinnacle of naturalist achievement. Other representative works are Dreiser's Sister Carrie, London's The Call of the Wild, Norris's McTeague, and Crane's The Red Badge of Courage.