Starting with an overview of theoretical approaches to emotion from an activity-oriented stance, this article applies Vygotsky's three general principles of development, sign mediation, and internalization to the development of emotional expressions as a culturally evolved sign system. The possible twofold function of expression signs as a means of "interpersonal" regulation and "intrapersonal" regulation predestines them to be a mediator between sociocultural and psychological processes in the domain of emotions. The proposed internalization theory of emotional development transfers Vygotsky's theory of the development of speech and thinking to the development of expression and feeling. Three stages of emotional development are described and underpinned by empirical studies: (a) the emergence of enculturated expression signs and related emotions from precursor emotions of newborns in the interpersonal regulation between caregivers and children during early childhood, (b) the emergence of intrapersonal regulation of emotions out of their interpersonal regulation by using expression signs as internal mediators that starts from preschool age onward, and (c) the internalization of emotional expression signs and the emergence of a mental plane of emotional processing. (Contains 1 figure and 2 footnotes.)