In his book "The Expression of the Emotions in Man and Animals," Charles Darwin (1872/1965) defended the argument that emotion expressions are evolved and adaptive (at least at some point in the past) and serve an important communicative function. The ideas he developed in his book had an important impact on the field and spawned rich domains of inquiry. This article presents Darwin's three principles in this area and then discusses some of the research topics that developed out of his theoretical vision. In particular, the focus is on five issues--(a) the question of what emotion expressions express, (b) the notion of basic emotions, (c) the universality of emotion expressions, (d) the question of emotion prototypes, and (e) the issue of animal emotions--all of which trace their roots to Darwin's discussion of his first two principles. (Contains 1 footnote.)