Realism and Naturalism emerged in American literature during the late 19th century, reflecting the nation's rapid industrialization and social changes. These movements rejected Romanticism's idealism, instead focusing on everyday life and the impact of environment and heredity on human behavior. Key authors like Mark Twain, Henry James, and Stephen Crane used detailed descriptions and vernacular language to portray ordinary people's struggles. Their works explored themes of social inequality, determinism, and psychological complexity, shaping American culture and inspiring social reform.