Organisms possess a special unity that biologists have long recognized and that cries out for explanation. Organs and collectives also have their own related kinds of unity, so what distinguishes the unity of the organism? I argue that only substantial form, a central plank of hylemorphic metaphysics, can provide the explanation we need. I set out the idea that whilst organisms possess substantial form, organs abtain the substantial form of the organisms they belong to, and collectives contain the substantial forms of their organismic members. I consider a number of difficult cases, including lichens, biofilms, cellular slime moulds, and plasmodial slime moulds, arguing that none of them pose a serious threat to the threefold distinction between organ, organism, and collective. I conclude by arguing that two prominent, alternative unity principles for organisms do not work, thus giving indirect support to the need for substantial form
