Many think that reality is structured such that some beings are more fundamental than others and characterize this structure in terms of 'grounding.' Grounding is typically regarded as explanatory and as exhibiting certain order-theoretic properties: asymmetry, irreflexivity, and transitivity. Aristotle's notion of ontological priority, which inspired discussions of grounding, also has these features. This Element clarifies Aristotle's discussions of ontological priority, explores how it relates to other kinds of priority, and identifies important connections to metaphysical grounding. Aristotle provides numerous examples that appear to impugn ontological priority's order-theoretic coherence. This is Aristotle's “Priority Problem.” But Aristotle has an independently motivated solution that eliminates the threat from each of the apparently problematic examples and explains why such examples are ubiquitous. The author argues that a ground-theoretic analog of Aristotle's solution to the Priority Problem addresses recent challenges to grounding.