In Leibniz's mature writings we find evidence of three distinct models of the universe. Most basically, there is the system of monads. According to this theory, reality consists exclusively of soullike substances and their harmoniously related perceptions. Alongside this is a model of the universe as understood from the point of view of physical theory, an account that describes extended bodies moving and interacting in accordance with the laws of mechanics. Finally, there is a model that merges aspects of both of the two preceding theories. According to the doctrine of panorganicism, nature is everywhere composed of organic creatures, each constituted from a soul or soullike form and an organic body specifically adapted to its needs. Given that each of these theories is advanced as a comprehensive model of the universe, it is easy to imagine them coming into conflict with one another. Leibniz is clear enough about the derivative status of the second model – that of physical theory – that there has not generally been a problem about its consistency with the other two models. Still, questions remain as to what exactly its relationship to them is. The connection between the other two models – the monadic and panorganic – raises deeper problems. I have argued that the doctrine of panorganicism is presupposed by Leibniz's account of the relatedness of monads within the best of all possible worlds.