The problem of the interpretation of Private Law concepts appeared with the born of Tax Law as a legal branch. In fact, the intercessions between Tax Law and Private Law always created great theoretical discussions with undeniable practical consequences. This thesis aims to investigate how the concepts of Private Law used by the Federal Constitution to define the taxing powers should be interpreted. For this project, we adopt, as method, the research of doctrinal documents, legislation and judicial decisions. We start inquiring the structure of concepts, and we face the classic Aristotelian theory with the prototype theory, developed from empirical research in cognitive psychology. Adopting the contributions of prototypical theory, we try to reflect if it is still useful the distinguishing between types and concepts, as it is done by some tax scholars. We also examine the content and scope of the tax typicality principle. Then, we study the didactic autonomy of Tax Law and the possibilities and limits of changing the meanings of concepts of Private Law. In reference of the constitutional taxing powers rules, we inquiry if the article 110 of National Tax Code solved all problems concerning the interpretation of Private Law concepts. We critically examine the expansion of taxing powers by an interpretative method, supposedly supported by the ability to pay principle. The supremacy of Constitution and its hierarchical rules do not allow that Private Law concepts can be changed by the tax rules, just to increase the taxing power, but it is not a barrier to a legislative or interpretative evolution of these concepts.