This article focuses on human-computer interface design based on the skills of human conversation. The metaphor of face-to-face conversation has been applied to human-computer interface design. Some of the features of human-human conversation being implemented in this new genre of embodied conversational agent. Embodied conversational agents also bring a new dimension to discussions about the relationship between emulation and simulation, as well as to the role of foundational principles in interface design. Embodied conversational agents represent a new slant on the argument about whether it is wise to anthropomorphize the interface. Researchers should be encouraged to develop embodied conversational agents to address the adequacy of their theories of human behavior when implementing effective interfaces. Properties of human conversation which had to be modeled for the systems are synchronization, function rather than behavior, multithreadedness and entrainment. Another factor motivating interface designers to develop embodied conversational agents is the increasing computational capacity in many objects and environments beyond the desktop computer, such as smart rooms and intelligent toys, in environments as diverse as military battlefields and children's museums.