The article aims to define the scope and theory of international security studies as a sub-discipline of international relations. The main idea is that studying the security of an entity, whatever it may be, is only possible within the framework of international relations as a science offering explanatory theories and models. In light of this, the article discusses the following issues: firstly, the structure of international security studies as a research field, taking into account the relevant level of analysis, i.e. entity/structure; secondly, a processual approach to the issue of security; and thirdly, spatial aspects of security