Pedagogical agents are on-screen characters that help facilitate learning in a virtual or mixed-reality setting. The research on pedagogical agents has shifted focus since their creation, from understanding their underlying principles, identifying their potential roles and usage, and recently finetuning their design for applied practice. This paper explores the history of pedagogical agents and summarizes the current state of knowledge. Once the current state of pedagogical agents is established, barriers and opportunities are discussed to suggest how researchers can best improve pedagogical agents and their implementation.