It is important for agents to communicate and interact with each other, especially if they are part of the same multi-agent system. In most cases, different agents are working collaboratively towards the same goal. They need to talk to each other, share tasks, exchange results etc. Here, it is important that agents understand each other; for example, they need to speak the same language or be able to translate and understand the language spoken by other agents. Ontologies are used to establish effective communication between different agents. Ontologies specify the terms used in agents' communication and provide the exact meaning of those terms relative to other ontology terms and within a specific context. Ontologies provide the agent with the domain knowledge and enable it to function intelligently. In this chapter, we will introduce ontologies. We will provide a definition of ontology and explain associated terminology such as ontology commitments, ontology representation, ontology classification; we will give a formal description of ontologies and ontology design criteria. © 2009 Springer-Verlag Berlin Heidelberg.