If there is a domain of social reality in which the positive and the normative, the facts and the values, are most entangled, it is in the category of social problems. Curiously, the ontology of social problems is a topic that has been radically ignored by the already extensive literature on 'social ontology'. This chapter attempts to partially remedy this situation, by providing a few guiding lines about what an 'ontology of social problems' should contain, and also by suggesting that social ontological theories might gain from considering social problems as a kind of fundamental ontological principle.