Since the late 1990s, computer-generated images, known as pedagogical agents, have been a medium of interaction within computer-based environments. Whether the interaction is reciprocal or one-way, pedagogical agents require the user to anthropomorphize the image as an entity that is lifelike and possesses a persona. With the advancement of technology, researchers have had the opportunity to design agents that perform or interact with humans in various ways. However, these advancing features of today’s pedagogical agents are different from human instructors and require specialized design considerations in theory and principle. This article will examine the various types of pedagogical agents and the key theoretical underpinnings of agents in computer-based environments.