In this paper we concentrate on the use of public ontologies in agent discovery, interoperation and communication. The rest of the chapter is organized as follows: section 1.2 concentrates on the contribution of ontologies to agents' social activities. We will show that ontologies are essential to the description of agents' "social interfaces", and in particular the description of agents' capabilities which are needed for agent discovery in a MAS. In section 1.3 we describe ontologies for agent communication and show how ontologies contribute to mutual understanding of agent messages and interactions. Section 1.4 We will then conclude highlighting some of the open challenges.