In this article, we discuss the meaning ofpersonality and its role in socially intelligent multiagent systems. After examining the reasons behind the current trend toward endowing software agents with personality, we introduce our notion ofpersonality as a combination of traits and attitudes. We characterize what we consider to be two basic elements of any cooperation activity (delegation and help), and we show how they can be diversified in relation to the agent's level ofautonomy and cooperativeness. We then describe how we formalize these forms of delegation and help, in GOLEM, a multiagent cooperation testbed, and we outline how these traits and attitudes can be organized into reasonable personalities and interesting interactive situations. Finally, we show how, in GOLEM, these traits and attitudes are involved in deciding what to do proactively or in response to other agents' social action, and in reasoning about other agents' minds.