More than 20 years have passed since B. Buzan, O. Wæver and J. de Wilde published their “Security: A New Framework for Analysis”, which has become a classic in the discipline of security studies. Although Russian scholars increasingly attempt to use the securitization theory’s conceptual apparatus in their research, the knowledge of the theory itself remains rather fragmentary. The overwhelming majority of existing papers refer to the so-called Copenhagen School’s (CS) intellectual heritage, while more comprehensive approaches and recent studies remain almost unknown among Russian scholars. The author attempts to fill this gap. This article is first in line of a series of studies, entirely devoted to the phenomenon of securitization: from the earliest milestones to the modern stage of development of the theory. The paper examines the theoretical and philosophical premises, as well as the ideas and assumptions of the securitization theory, first formulated by O. Wæver in the late 1980s. The author refers to the original texts of the main figures of the CS: O. Wæver and B. Buzan, conceptualizing the history of the concept of securitization and immersing the reader into the atmosphere of security studies field at the end of the 20th century. As a result, it becomes possible to determine the key elements of the early theory of securitization: security as a speech act, national security as a main focus of study, post-structural realism as a research agenda of O. Wæver, and the idea of security as a negative meaning. The article concludes that despite the shortcomings of the early theory of securitization noted by many critics, it was based on a valuable and fruitful idea - an attempt to go beyond the notion of security as an absolute good or a metaphysical entity, which was typical of traditional and many alternative approaches to the definition of security.