The concept of ontological security has grown extensively in the International Relationsliterature, owing to the new explanations it generates for states’ security- and identity-relatedbehavior. In the process, however, the concept has become sprawling, vague and incoherent,due to the multitude of different understandings of the concept. To improve the concept’sadequacy, counter the risk of conceptual stretching, and provide the foundation for a commonresearch agenda, this essay constructs a typology to divide and classify the ontological securityliterature in IR. The typology’s main contribution is the dimension of ontological insecuritymechanisms, understood as the different ways that the ontological security of an agent can bethreatened, and the different types of existential anxieties that follow. To test and illustrate thistypology, the essay conducts an empirical case study of Russia’s engagement in the conflict inSyria since 2015. The results strongly indicate the presence of all ontological insecuritymechanisms, thus clearly implying that Russian behavior in Syria is driven by ontologicalsecurity concerns. The findings also demonstrate the typology’s usefulness and fruitfulness inmore closely specifying the nature of the ontological insecurity in particular cases.