The research presented in this PhD-thesis deals with the problematic concept of Hellenistic philosophy and the related actual problems of periodization and terminologization of the PostAristotelian period of Ancient philosophy. In the Introduction, the author briefly provides the context, reason and purpose of his research: context is a scientific revolution that resulted in the construction of a new image of Ancient philosophy in which the narrower concept of Hellenistic philosophy has prevailed; the reason is that the concept of Hellenistic philosophy has undergone various changes throughout history because it has caused doubts from the very beginning and still causes them; while the purpose is a better historical understanding of the concept of Hellenistic philosophy: a deeper insight into the meaning of this concept and finding the periodization terminology that would have better foundation and functionality in the context of the emerging image of Ancient philosophy. First we explore the traditional notion of Hellenistic philosophy that has long prevailed thanks to the influence of German historiography. Namely, it is well known that the creator of the historiographical notion of Hellenism was Johann Gustav Droysen, but it is less known that the notion of Hellenistic philosophy came into wider use only with the influential works of Wilhelm Windelband. Contextual analysis of Windelband's philosophical manuals has revealed that he used the notion of Hellenistic philosophy in rather a different sense from the one that prevailed in recent times when it was limited to the pre-imperial period of PostAristotelian philosophy. It has been shown that his notion of Hellenistic philosophy was based both on Droysen's understanding of Hellenism as a synthesis of East and West, and on the extensive reception of literature dealing with the study of Alexandrian culture and the Hellenistic background of Christianity. It was pointed out that such research came mostly from Hegel's students, and also that Droysen's understanding of Hellenism has its roots in Hegel. As Windelband called the entire Post-Aristotelian period Hellenistic-Roman philosophy, but mostly used the term Hellenistic philosophy for its religious part belonging to the Roman Empire, his periodization and terminologization naturally caused doubts and it was found that at the beginning of the 20th century the scope of the term Hellenistic philosophy varied significantly and was therefore unclear and undefined. The fact that the traditional notion of Hellenistic philosophy, after all, usually meant all Post-Aristotelian philosophy was explained 311 both by the great influence of Windelband's History of Philosophy and by the adoption of such a concept by the most prominent scholars of the Post-Aristotelian period of the early 20th century: August Schmekel and Karl Praechter. The most extensive part of the dissertation is devoted to the analysis of Praechter's periodization of Post-Aristotelian philosophy. Finding his conception of Hellenistic philosophy theoretically better grounded than that developed in recent times when it seeks to conform to the conventional boundaries of Hellenistic period according to the criteria of political history, the author enters into a critical examination of Praechter's periodization taking into account the results of recent research that resulted in the rehabilitation of PostAristotelian philosophy. Critically analysing its external and internal boundaries, the author sought to find a well-founded and functional synthesis of traditional and modern periodization. The second part of the dissertation takes a closer look at the concept of Hellenism in order to clear up doubts about its application in the periodization of Ancient philosophy. The causes that made this notion unclear are pointed out, and the solution is seen in the return to Droysen's original notion of Hellenism. His original conception of Hellenism as a GreekOriental synthesis is found adopted by Hans Jonas and author on this ground proposes a new periodization which, although based on different terminology, in its main parts proves to coincide with Hegel's division of Ancient philosophy.