American states are subject to three constitutional restrictions that are relevant to environmental regulation. The first is called the dormant commerce clause doctrine. It prohibits states from engaging in regulation that discriminates against interstate or foreign commerce or that unduly burdens such commerce. This doctrine is analogous to EU mandates governing the free movement of goods, but has distinctive rules. The second is statutory preemption. Under the Supremacy Clause of the U.S. Constitution, a state law that conflicts with a federal statute is invalid. Preemption doctrine also invalidates state laws that interfere with the goals of federal statutes less directly. There is no equivalent of the subsidiarity concept in U.S. constitutional law. The third constitutional restriction is especially pertinent to EU-California cooperation. Under the doctrine of foreign policy preemption, a state law is invalid if it invades the exclusive control of the national government over foreign policy. Unlike EU member-states, American states do not retain the power to engage in their own foreign policy. The Supreme Court has issued two recent opinions on this subject, which are generally regarded as unclear in their exact meaning but as significantly expanding this doctrine. This paper will describe the constitutional doctrines and discuss how cooperative agreements can be designed to avoid constitutional problems.