I first examine instances of grounding in epistemology, arguing that the grounding relation generates Aristotle’s regress problem: if every justified belief required justification from another justified belief, an infinite regress would result. This has significant consequences for the debate among foundationalists, coherentists, and infinitists. Grounding is asymmetric and so does not admit of circularity; this rules out coherentism, which appeals to circular justification. If Grounding is also well-founded, in that it bottoms out in fundamental truths, this rules out infinitism, which embraces the regress. Grounding in epistemology thus offers a strong basis for foundationalism. I turn next to the epistemology of grounding, arguing that conceptualist theories of a priori knowledge, like those defended by David Chalmers, Paul Boghossian, and Christopher Peacocke, cannot account for a priori grounding knowledge. These views attempt to explain our a priori knowledge in terms of our understanding of concepts, so they must explain our grasp of a priori instances of grounding in the same way. But the structure of our concepts is not isomorphic with the structure of grounding. My preferred, essentialist view explains our grasp of a priori instances of grounding in terms of knowledge of essences. In contrast to the conceptualist view, the essentialist view can capture all of our commonsense a priori judgments about grounding.