The Expression of the Emotions in Man and Animals is Charles Darwin's third major work of evolutionary theory, following On the Origin of Species (1859) and The Descent of Man (1871). Initially intended as a chapter in The Descent of Man, The Expression grew in length and was published separately in 1872. This book concerns the biological aspects of emotional behavior, and Darwin explores the animal origins of such human characteristics as smiling and frowning, the shrugging of shoulders, the lifting of the eyebrows in surprise, and the baring of teeth in an angry sneer. A German translation of The Expression appeared in 1872; Dutch and French versions followed in 1873 and 1874. Since its first publication, The Expression has never been out of print, but it has also been described as Darwin's "forgotten masterpiece"; psychologist Paul Ekman has argued that The Expression is the foundational text for modern scientific psychology.

Charles Robert Darwin was an English naturalist, geologist and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended from a common ancestor is now generally accepted and considered a fundamental concept in science. In a joint publication with Alfred Russel Wallace, he introduced his scientific theory that this branching pattern of evolution resulted from a process he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding. Darwin has been described as one of the most influential figures in human history and was honoured by burial in Westminster Abbey.