Pancomputationalism (PC) is the sensational view that every physical system—including the universe as a whole—is a computing system. Ontic PC is the increasingly popular version of PC according to which the physical universe is fundamentally computational. According to ontic PC, there is a fundamental level of physical reality at which the one and only fundamental computation performed by each physical system can be identified. In addition, that fundamental computation is all there is to the nature of a physical system. Proponents of this view are compelled by the remarkable capacities of computers to simulate physical processes either approximately or exactly, and by the unifying descriptive power of mathematics in both computer science and physics. In this chapter, we analyze arguments for ontic PC and find them far less clear, complete, and plausible than do their proponents.