Computer-mediated communication (CMC) renewed the interest for collaborative learning. Empirical findings show that collaborative learning is efficient, but only under some conditions. These conditions are not guaranteed by the use of CMC tools. It would be an over-generalisation to expect any type of computer-supported collaborative learning (CSCL) to be efficient. It is also difficult to translate the conditions identified in face-to-face collaboration into conditions for success of distance collaboration, because too many other factors separate face-to-face from distance interactions. However, the analysis of these conditions reveals some mechanisms which explain the effects of collaborative learning. This contribution reviews the mechanisms which have been proposed and considers to which extent these mechanisms could be present in CSCL.