Orthodox Churches have been in full membership of the WCC since its foundation (Amsterdam 1948) but before that they had already participated in various ways in “Faith and Order” and “Life and Work”, the movements from the fusion of which the World Council of Churches originated. This presence - often characterized by a certain tension with other member churches, mostly of protestant origin - has become ever more important, because of the steady growth in the number of Orthodox Churches participating in the WCC, and also because of the quality of the Orthodox contribution. After an historical summary, some problems that influence the quality of the Orthodox presence are presented, namely: a) the ecclesiological question; b) the basic article of the WCC’s Constitution; c) the tension between theological requirements and social involvement; d) the problem wich has recently been put forward more insistently of proportionate Orthodox representation in the structures of the WCC. There follows an account of the Orthodox evolution of its own presence in the WCC and of the positive contribution given to the search for unity. In fact, this presence has been substantially useful for the WCC and the life of the Orthodox Churches themselves. At the recent General Assembly in Vancouver (1983) there emerged the desire of the Orthodox Churches to stress and define more clearly their own contribution in the overall activity of the WCC. In fact, the tension often caused within the WCC, by the Orthodox presence with its own theological demands (tradition of the undivided Church respect for dogmatic decisions. sacramentality, liturgy, etc.) stimulates an internal dynamism in the theological debate and ecumenical action (pp. 153-194).