This essay offers a programmatic statement for a realist theory of law. Although I have been influenced by (and written about) the work of earlier American, Scandinavian, Italian and other legal realists, this is not an essay about what others have thought. This is an essay about what I take realism about law to mean and what its theoretical commitments are; I shall use other realists to sometimes illustrate the distinctive positions of a realist theory of law, but will make clear where I depart from them. A realist theory of law involves both a “realist” and a “naturalistic” perspective on law. Let me explain how I understand these perspectives.