This essay discusses and criticizes the claim that normative political theory can be (justifiably and fruitfully) divided into two parts—a part having to do with ideal theory which assumes full compliance and abstracts away from issues having to do with implementation and, contrasting with this, a nonideal part having to do with implementation and with rules and institutions appropriate for conditions of partial compliance. On this conception of ideal theory, empirical facts about human behavior and motivation, connected to issues surrounding compliance and implementation, are irrelevant to ideal theory, although such facts can be relevant to the nonideal part of normative theory. I argue against this conception, holding instead that such empirical facts are relevant to most or all of normative political theory, including “fundamental” normative principles.