Classical Trinitarianism holds that every action of Trinity in the world is inseparable. That is, the divine persons are equally active in every operation. But then, in what way did the Father create the world through Christ? How can only the Son be incarnate, die, and be resurrected? Why does Christ have to ascend before the Spirit may come? These and many other questions pose serious objections to the doctrine of inseparable operations. In the first book-length treatment of this doctrine, Adonis Vidu takes up these questions and offers a conceptual and dogmatic analysis of this essential axiom, engaging with recent and historical objections. Taking aim at a common “soft” interpretation of the inseparability rule, according to which the divine persons merely cooperate and work in concert with one another, Vidu argues for the retrieval of “hard inseparability,” which emphasizes the unity of divine action, primarily drawing from the patristic and medieval traditions. Having probed the biblical foundations of the rule and recounted the story of its emergence in nascent Trinitarianism and its neglect in modern theology, Vidu builds a constructive case for its retrieval. The rule is then tested precisely on the battlegrounds that were thought to have witnessed its defeat: the doctrines of creation, incarnation, atonement, ascension, and the indwelling of the Spirit. What emerges is a constructive account of theology in which the recovery of this dogmatic rule shines fresh light on ancient doctrines.