Does the Constitution limit the extent to which Congress can grant discretion to other actors? The traditional nondelegation doctrine says yes, though advocates of the doctrine strongly disagree about the source of that principle and the location of the line between permissible and impermissible discretion. A number of modern scholars and judges, however, doubt whether the Constitution contains any such principle. This article demonstrates that the Constitution constrains Congress's ability to grant discretion to other actors through the requirement that laws for carrying federal power into execution must be "necessary and proper." The words "necessary" and "proper" have distinct constitutional meanings. The word "necessary" describes a causal connection between means and ends that is less strict than the standard of indispensability urged by founding-era figures such as Thomas Jefferson but that is considerably stricter than the "rational basis" standard exemplified by modern law. The word "proper" refers to consistency with background principles of separated powers, federalism, and individual rights that infuse and define the Constitution's specific architectural arrangements. Excessive grants of discretion can be unnecessary, improper, or both, and can thus exceed Congress's enumerated powers. Far from being, as Eric Posner and Adrian Vermeule have recently argued, a "neurotic burden" on the legal system, the nondelegation doctrine has a firm constitutional foundation.