Since California passed the first medical marijuana legislation in 1996, 22 other states and the District of Columbia have followed suit. Colorado and Washington legalized recreational use of marijuana in 2012, and last year saw Oregon, Alaska, and D.C. join them. However, under federal law, marijuana remains a Schedule I controlled substance and illegal for any use, whether medical or recreational. As a result, numerous courts across the country have been asked to answer the question of whether federal law making marijuana use or possession illegal preempts state law decriminalizing or legalizing it. In other words, when it comes to marijuana, does federal law trump state law? Federal preemption occurs when a state law or regulation conflicts with a federal law or regulation. Article VI of the U.S. Constitution, known as the Supremacy Clause, provides that the laws of the United States have supremacy over state constitutions and laws, so that if a state law is in conflict with federal law, federal law trumps. However, what might seem to be a clear case of preemption isn’t always. There are three main types of preemption – express, field, and conflict. Courts have agreed that neither express preemption (where Congress has expressly prohibited states from regulating a particular area via legislation) nor field preemption (where Congress has enacted laws that are so pervasive there is no room for the states to supplement) apply to the area of medical marijuana. That leaves conflict preemption, which is the most complex type of preemption, and the most difficult to determine. Most people would look at a state law allowing the medical use of marijuana as being in direct conflict with the federal law prohibiting the use or possession of marijuana for any reason. However, that has not been the finding in the courts that have addressed the issue. The outcome rests on the distinction between decriminalization, i.e., removing criminal penalties for possession and use, and legalization, i.e., permitting the recreational use of marijuana. Both Arizona and California have laws that decriminalize certain acts under state law related to the medical use of marijuana, and in both states the courts have determined that the laws decriminalizing the medical use of marijuana in those states do not conflict with federal law as the state laws do not pose an obstacle to federal enforcement of federal law. Essentially, what the courts said was that decriminalizing use and possession for the medical use of marijuana does not prevent federal law enforcement officers from enforcing federal law, it only prevents prosecution under state law. Therefore, state law was not preempted in those cases. However, the Supreme Court of Oregon reached a different decision. The Oregon medical marijuana provisions both legalized use or possession of marijuana for medical use and decriminalized use or possession of marijuana for medical use. As a result, the court found that the provision legalizingpossession of marijuana presented an obstacle to the enforcement of federal law and created a conflict.[ii] “To the extent that [Oregon law] affirmatively authorizes the use of medical marijuana, federal law preempts that subsection, leaving it ‘without effect.’”[iii] However, the court also stated that they did not hold that the other sections of the Oregon Medical Marijuana Act decriminalizing the use of marijuana for medical purposes were preempted.[iv] As can be seen from these cases, and the Oregon case in particular, the distinction between decriminalization and legalization is crucial to the outcome in these cases. Once a law is found to be preempted, it is no longer in effect. With the emergence of marijuana legalization at the state level, the question of preemption is certain to become an important one, and one that will most likely result in the preemption of those laws legalizing the use and possession of marijuana. At this time, however, it appears that the federal government’s focus will remain on trafficking of marijuana, not recreational use in states where it has been legalized.