This chapter traces naturalism, a radical outgrowth of realism and one of the earliest movements in modernist theatre, beginning with its first articulations by Émile Zola and his French contemporaries through to manifestations, variations, and subversions of naturalist ideas across Europe, the United States, China, and India. Based in scientific epistemologies and a rejection of aesthetic idealism, naturalism introduced still potent innovations in dramatic form, scenography, audience experience, and the division of labour in theatre. Through confronting depictions of character and agency as fundamentally shaped by physiological, hereditary, and environmental forces, naturalism paved the way for later reformist theatre while seeding subsequent modernist movements that rebelled against its physicalist and materialist accounts of human experience.