This chapter introduces and surveys the emerging agent-centered artificial intelligence (AI), and highlights the importance of developing theories of action, learning, and negotiation in multi-agent scenarios such as the Internet. It examines in detail the main functionalities software systems would display in a social, agent-centered AI or, in other words, the principles of behavior of the new AI. Approaches to multi-agent behavior differ mainly in regards to the degree of control that the designer should have over individual agents and over the social environment, that is, over the interaction mechanisms. Research on machine learning has been mostly independent of agent research and only recently has it received attention in connection with agents and multi-agent systems. Agent-based applications have enjoyed considerable success in manufacturing, process control, telecommunications systems, air traffic control, traffic and transportation management, information filtering and gathering, electronic commerce, business process management, entertainment, and medical care.