This book is the first in a two-volume project that provides the first systematic interpretation of Martin Heidegger’s fundamental ontology as a critical social ontology. It argues that fundamental ontology is, at its core, an emancipatory enterprise in that it is animated by an intent shared with critical theory, viz. to make philosophy concrete for the sake of individual and collective freedom. Focused on Division One of Being and Time, this volume maintains that the possibilities of individual emancipation and self-actualization are inextricably bound to their environmental and intersubjective preconditions. An adequate ontology of the self, therefore, is possible only as a social ontology. To fulfill its emancipatory aims, however, it is necessary to show that the fact that individuals are always already appropriated by society does not preclude the possibility of authentic self-appropriation. Hence, this volume critically reconstructs the core commitments of the ‘appropriative approach’ to social ontology and distinguishes between the foundational and peripheral aspects of fundamental ontology. The final part addresses important criticisms that have impeded a fruitful dialogue between fundamental ontology and critical theory to clear the ground for a fresh re-appraisal of the former as an emancipatory project.