In this study, I intend to contribute to a better understanding of what kind of thing an obligation is and what its defining features are. Central to the conception I will put forward is the idea of obligation as having two essential aspects: one of these lies in the internal connection of obligation with moral practical reasons and is accordingly rational and moral; the other one instead lies in the conceptual link between obligation and requiredness, or mandatory force. In combination these two aspects, which interlock to form what I would call the duality of obligation, frame obligation as a rational and morally justifiable categorical requirement.