Hylomorphism is a theory that has regained prominence in contemporary metaphysics, explaining the unity of composite material objects by appeal to a special metaphysical part of those objects: a structure or form. I begin by reviewing three prominent Neo-Aristotelian hylomorphic theories: that of Kit Fine, Kathrin Koslicki, and William Jaworski. Each of the three thinkers hold that any genuine material object with parts is a ‘substance,’ and that structures account for the existence of substances. Despite taking radically different positions on the nature of material objects and form’s role in accounting for their composition, they all hold a controversial principle: substances can have other substances as proper parts. For example, if a water molecule is a substance, the hydrogen and oxygen atoms that compose such a water molecule remain substances when they are that molecule’s proper parts. I argue that the principle that substances can have other substances as proper parts poses serious problems for the coherence of hylomorphism. However, if it is false that substances can have other substances as parts, hylomorphism can be shown not only to be a plausible theory, but the existence of entities functionally equivalent with substantial form is entailed by plausible assumptions about material composition. Aside from rejecting the principle that substances can have other substances as parts, these plausible assumptions are: that there are material objects with parts and that there are some objects that are not parts of other objects. I propose Thomas Aquinas’ metaphysics of material objects as an exemplary hylomorphic theory holding there can be at most one substantial form in each substance. His theory both illustrates the functional role of substantial forms in the composition of objects, and gives plausible answers to puzzles that seem to follow from rejecting the principle that substances have other substances as parts. After examining Aquinas’ metaphysics, I argue that Zhu Xi, a Song dynasty Confucian metaphysician, should be characterized as having developed a non-Aristotelian hylomorphism. Utilizing Zhu Xi’s theory, given his independence from the Aristotelian tradition, I construct an argument that theories of material composition, given the aforementioned plausible assumptions about the nature of composition, are committed to the existence of entities like substantial forms. I present Zhu Xi’s metaphysics in contrast to that of Kit Fine, arguing that whatever it is in virtue of which a substance is composed of its proper parts should be a metaphysical part of those substances, not a mind-dependent intensional principle. Consequently, accepting a framework in which there are mereologically composite material objects entails, on relatively weak assumptions, that these objects have substantial forms. Rather than saddle our metaphysics of material composition with unnecessary presuppositions, this fact that substantial forms play an implicit role in our metaphysics is an important feature of those theories. Hylomorphism is then not merely a plausible or appealing solution to problems of material composition, but a position entailed by any coherent metaphysics of ordinary material objects.