In open MAS it is unlikely that agents developed by different communities share the same ontologies. This lack of homogeneity can make communication very difficult, as agents may fail to understand properly the messages they receive. Some form of agreement over the content of the message might be needed. In this talk I will propose some reference ideas that I hope will help me in my research in the field of mutual understanding between agents. I will also present a small example to clarify some of the concepts introduced.