In a growing number of states, medical marijuana occupies a dual legal status. The federal Controlled Substances Act (CSA) classifies marijuana as a Schedule I drug, signifying that it has both a high potential for abuse and no acceptable medical use. Patients and physicians are thus subject to the same federal criminal penalties as any other individuals who produce, distribute, or possess marijuana. By mid-2009, however, fourteen states had decided to permit medical marijuana under certain circumstances. Through voter initiatives or legislation, these states have exempted patients and physicians from prosecution for violating state laws governing the use, possession, or cultivation of marijuana. The disparate legal regimes have generated much litigation, including two cases that reached the U.S. Supreme Court. Both cases rejected challenges to federal enforcement of the CSA in California, the state with the most liberal provisions on medical marijuana. Under the Court decisions, federal authorities could continue to take action against patients, physicians, and others who were protected under state medical marijuana laws. Two recent legal developments are more favorable to states seeking to permit medical marijuana, however. First, U.S. Attorney General Eric Holder announced that the federal Drug Enforcement Agency would limit future raids and other enforcement activity to individuals violating both federal and state criminal law. This means that people complying with state medical marijuana laws will no longer be targeted by federal law enforcement officials. Holder said that under his leadership, the agency would focus its efforts on large-scale commercial marijuana operations. (1) In a second development with implications for state marijuana laws, the U.S. Supreme Court declined to review a court decision upholding legislation implementing California's medical marijuana law. In 1996, California voters approved Proposition 215, designed to ensure that seriously ill Californians have the right to obtain and use marijuana for medical purposes where that medical use is deemed appropriate and has been recommended by a physician. (2) Seven years later, the California legislature enacted the Medical Marijuana Program Act to supplement the earlier law. (3) The 2003 law requires counties to participate in a system that enables patients and caregivers to register and obtain identification cards protecting them from arrest for violating state marijuana prohibitions. County versus State versus Federal Authority California officials unhappy with the state's permissive stance on medical marijuana balked at the mandate to create the identification system. San Diego and San Bernardino counties brought a lawsuit seeking a judicial declaration that they were not required to comply with the state's mandate. According to the plaintiffs, the California law was preempted by the CSA, which fails to exempt medical marijuana from the usual criminal penalties. The U.S. Constitution gives Congress the power to supersede state law. Over the years, courts have developed principles for determining when federal law preempts state law. Congressional purpose is the central consideration in resolving preemption claims. The easiest case arises when Congress includes in the legislative text an explicit provision on whether the law is intended to preempt state rules. In other cases, courts consider factors such as whether the state law under challenge presents an obstacle to achieving Congress's objectives in enacting the pertinent federal law. In 2008, a California appellate court ruled against the counties, holding that preemption failed to bar implementation of the California identification card program. (4) As the court noted, the CSA itself states that it is not intended to preempt state laws unless there is a positive conflict between [the CSA and state law] so that the two cannot consistently stand together. …