Recent research indicates that people respond socially to computers and perceive them as having personalities. Software agents especially are human-machine interaction artifacts that embody those qualities most likely to elicit social responses: contingent behavior, fulfilling a social role, and using language. People may perceive agents as one or many discreet social entities, each with a personality that will shape people’s interactions with the machine. An industrial team’s research on and experience with the design of software agent person-alities is described, in the context of related social science and computer science research. The conclusions from this experience are summarized as guidelines for future agent developers.