This chapter addresses ontology, which is the branch of philosophy concerned with the nature of being. As a branch of metaphysics, ontology is mainly concerned with the modes of existence of different entities (tangible and intangible). Every subdiscipline in the social sciences relies on an ontology that defines which elements really matter when it comes to explaining the phenomenon they set out to elucidate. A specific branch of ontology is devoted to the modes of existence of social phenomena: social ontology. Two main positions emerge: realism and constructivism. Scientific realism assumes that social phenomena have an objective existence, independent of the subject. By contrast, constructivism claims that social phenomena have no objective existence and are a construction of the human mind. Its fundamental axiom is that, even if reality exists outside the subject’s perception, the subject cannot reach it without perceiving it. This implies the mediation of imaginary structures, which are provided by social groups. It is important to note, however, that many other positions exist apart from realism and constructivism.