Multi-agent systems (MASs) ([5,6,19]) are a new and promising area in the field of distributed artificial intelligence (DAI), as well as in the mainstream computer science. These systems are compound of relatively autonomous and intelligent parts, called agents. Even if we restrict ourselves to computer science, a word ‘agent’ has many meanings. In [19] agent is defined as: “a hardware or (more usually) software based computer system that enjoys the following properties: utonomy: agents operate without the direct intervention of humans or others, and have some kind of control over their actions and internal state;social ability: agents interact with other agents (and possibly humans) via some kind of agent-communication language;reactivity: agents perceive their environment (which may be the physical world, a user via a graphical user interface, a collection of other agents, the Internet, or perhaps all of these combined), and respond in a timely fashion to changes that occur in it