The aim of this paper is to make a brief overview of the understanding of the concept of substance established by philosophers in the rationalist period, where the focus will be set primarily on the understanding of such concepts as we find them in the works of Descartes and Spinoza. The Structure of this paper is based on the authors set framework, outlining the general characteristics of rationalism itself, followed then with contemporary, generalyknown definitions concerning „substance as such“ so that it is easier to enter into a deeper analysis of the topic before us.At the center of the discussion will, of course, be the problem of cognition and the definition of the concept of substance, which we will analyse individually, dividing the work into two parts; the first part dedicated to Descartes, and the second part dedicated to Spinoza.Of course, each of the mentioned rationalists philosophers carry with their systems a certain shortcoming or problem to which we must and will draw attention to, and present their attempts to solve their own problems or "holes" in their systems, aswell as analyse those problems with suggestions of contemporary interpretations. We simply cannot shake off the impression that each set of characteristics asserted by these two philosophers often entails various implications where interpretative space is created for possible criticism. Also, through the paper, concepts that are closely intertwined with substance will be placed under observation, namely the concepts of attribute, modes and finally the concept of God, which in some way still significantly retained the influence of medieval perception of substance.