This dissertation aims to analyze the notion of good in Thomas Aquinas' metaphysics. In order to do so, it will be inquired how Aquinas introduces this notion through a double characterization: good understood as one of the general notions of the being, i.e., a transcendental property, and good regarded as one of the proper divine attributes, i.e., a transcendent property. In this sense, it shall be analyzed, first, the general characteristics of the transcendentals and the way Aquinas asserts that the good, once it is convertible to the being, may be regarded as a transcendental. Secondly, it will be analyzed how Aquinas states that this notion may be said to be transcendent, provided that the good is one of the divine perfections. However, such a characterization of the good apparently brings out a contradiction: How can this notion be said to be a transcendental and, simultaneously, to be transcendent? In order to inquire this apparent contradiction in the characterization of the good, it is necessary to make explicit the way Aquinas preserves this double aspect by using the model of predication by participation. Thus, this dissertation aims to highlight the justification found by Aquinas to establish the relation between a transcendental property and transcendent one by making use of a specific model of predication by participation which, in this context, is taken as a metaphysical foundation to the double characterization of the notion of good. According to Aquinas, creatures are said to be good because they participate in divine goodness. As this study deals with the notions of predication and participation, it will be presented how the doctrine of transcendentals constitutes a peculiar model of philosophical analysis which has received, from Aquinas, a distinct treatment from the tradition before him and is not only the reception of the sources available to him.