The author attempts in this study to show the nature of Tradition as related to Scripture according to the theological points of view of D. Petavius and L. Thomassinus, outstanding figures of the so called positive school. Thomassinus was a follower of Petavius and both were authors, respectively, of extremely important works, which are titled Dogmata Theologica. The present study is concerned with these figures, concentrating especially on the theological methodology of both and on the perspective of their theological knowledge. I. Petavius represents an important landmark in the theology of Tradition, notably because of his decisive influence on the socalled Roman School of Theology; which would borrow, among other things, his classification of the traditions, going from there -above all through J. B. Franzelin- to the habitual theological teaching on the subject. To know his posture it is decisive to be familiar with his interpretation of the celebrated text of Saint Basil, which was included among the auctoritates of the Council of Trent. In this commentary, Petavius' position in regard to tradition is perfectly clear: Tradition has a double role in regard to Scripture: on one hand it clarifies and explains the truths obscurely expressed in SCripture, and on the other, it transmits truths which we could not otherwise know through Scripture alone. Petavius insists and affirms that there are truths of faith, not merelyrites and customs, which have come down to us through apostolic tradition. However, he does not defend the theory of sources, but affirms, nothing more -in response to the protestants- that we cannot know through Scripture alone all of revealed truth; that tradition is necessary. For Petavius, as for Thomassinus afterwards, there is no doubt -and his theological manner demonstrates this- concerning the foundation in Scripture of all the truths of the faith, a different matter than knowledge of these truths only through the Holy Bible. But Petavius is aware that the authentic problem does not rest on the two-fold questions of Scripture-Tradition; the root and center of the problem lies in recognizing or not the teaching power of the Church, which has as its mission the interpretation of Scripture and Tradition. II. Although Thomassinus reflects, relative to Petavius, more differences in his theological thinking than are habitually noted, there is substantial agreement between the two on the theme which we are concerned at present. According to Thomassinus, Tradition, in its deepest sense, is truth and supernatural life revealed and given by Christ to his Apostles and their successors and through them to the whole Church by the coming of the Holy Spirit: in this sense, Scripture along with the traditions constitute Tradition. This concept would, at a future date, have repercussions in the theology of J. A. Mohler and, through him, in the Roman School. But, in so far as it is distinct from Scripture, Tradition has a dual role: it formally completes Scripture, endowing it with its true sense; it also completes it materially, relative to the question the prostestants had given rise to: that is to say, we do not know all of the truths of the faith through Sacred Scripture alone, and consequently, we need Tradition. This fact leads the Catholic Church to give the same prestige to the one as to the other. But the key to the relation between Scripture and Tradition, for Petavius and Thomassinus as well, is the Church which, thanks to a special divine assistance, safeguards and transmits both Tradition and Scripture. III. By way of resuming, we might add that the study of these two great theologians does not permit leaving out the concept of completive or constitutive tradition, which is its very basis.