Computer supported collaborative learning occurs primarily by means of communication. Successful computer supported co-operation is only possible if communication among learners and between learners and teachers is appropriately supported. We describe communication on the basis of a context-oriented communication model that focuses on the dialogical communication and mediation of context. The model leads to an abstract identification of general communication tasks amongst collaborating learners and between them and their teachers. The resulting list of tasks can be generally used to derive requirements for the support of communication in collaborative learning environments. This is demonstrated with the features of the prototype system “KOLUMBUS”, which is used for experimental investigation in university courses. It realises the support of all identified communication tasks. The paper ends with a brief summary of results taken from experiences at a university seminar as part of a larger research work.