This article addresses the conceptual scope and comprehension of the unconscious category in Sigmund Freud's theory. The presentation of Freud's classical description of the mental apparatus is aimed to recall the theoretical solutions founding the next psychological approaches with regard to the unconsciousness phenomenon. Apart from its historical importance, return to this part of the psychoanalysis founder's work serves as a way to deepen the comprehension and understanding of the very unconscious category, which is indeed so fundamental for various schools of psychotherapy practice and performance. It is also consistent with the main thesis of Jacques Lacan, who insisted on and called for a return to reading of the original works of Freud. Therefore, this article bounds to disclose the unconscious category and its position in the psychoanalytical theory within the framework of the first as well as second topical description of the mental apparatus.