SUMMARY: From a review, above all, of the discourses of recent Popes and various figures in the Eastern Churches, of their common declarations and the documents arising from the dialogue, it is clear that the use of the term «sister Churches», from an original reference to the relationship among the Eastern Churches, has assumed an extended range to refer also to the relationship between the Churches of the East and of the West. A careful analysis of its doctrinal basis indicates, however, that the claimed identity of faith has not yet been realized between the Catholic Church and the Orthodox Churches, which still lack a full participation in the communion of the one Church. As they are in a condition which implies a woundedness in their existence as particular Churches, the Orthodox Churches are true Churches, but not in the «full sense of the term.» Rather, they are such in an «imperfect manner» or in «an analogous or participative sense». Equally imperfect and analogous, therefore, would be their condition as «sisters». For this reason, the use of this term in the area of ecumenism should avoid certain tendencies which are quite widespread today. In particular, it is inaccurate to consider the Catholic Church and the Orthodox Church as two parts of the one Church, two universal sisters, independent and interchangable, which would together constitute the one Church of Christ, held to exist in history no longer, but seen as a Church in fieri, found in the convergence and reunification of the sister Churches as desired and promoted through dialogue. Similarly, certain expressions should be avoided, also quite common in ecumenical parlance, which reflect this tendency, such as the unity of the Church and our two Churches. Moreover, in the area of ecumenism, it is erroneous to consider the Orthodox Church as a single Church and the Catholic Church as a particular Church. Concretely, the expression «sister Chur-ches» could be used also to refer to a particular Catholic Church, including that of Rome. But it is theologically unacceptable to use the term «sister Churches» to refer to the relationship between an individual particular Church, or groups of these, on one hand, and the Catholic Church in its entirety on the other, or to use it to refer to the relationship between the «Orthodox Church» - which as such does not exist - and the Catholic Church, understood in the universal sense, in which subsists the one Church of Christ.