  Socially interactive agents (SIAs) are no longer mere visions for future user
interfaces, as 20 years of research and technology development has enabled the
use of virtual and physical agents in day-to-day interfaces and environments.
This chapter of the ACM "The Handbook on Socially Interactive Agents" reviews
research on and technologies involving socially interactive agents, including
virtually embodied agents and physically embodied robots, focusing particularly
on the appearance of socially interactive agents. It covers the history of the
development of these technologies; outlines the design space for the appearance
of agents, including what appearance comprises, modalities in which agents are
presented, and how agents are constructed; and the features that agents use to
support social interaction, including facial and bodily features, those that
express demographic characteristics, and issues surrounding realism, appeal,
and the uncanny valley. The chapter concludes with a brief discussion of open
questions surrounding the appearance of socially interactive agents.
