Abstract: This chapter is primarily an exposition of the applicable constitutional doctrine on the enforcement of national law against subnational units in the US. It also offers some general observations about the underlying theory of federalism that generates US constitutional doctrine. In the US the question of the enforceability of national law against state governments is a matter of some theoretical interest but relatively little practical importance. The reasons for that situation are a combination of institutional and historical conditions, which the chapter refers to in more detail. For those outside the US, however, the primary message here is that the constitutional doctrine dealing with this sort of enforcement is quite limited in scope and importance, in contrast to its importance in systems whose constitutions create a less centralized version of constitutional federalism.