Last November, 2012, the people of Colorado and Washington voted to legalize the possession and use of small amounts of marijuana and to regulate how marijuana is produced and distributed in their States. These new laws are just the latest examples of the growing tension between federal and State marijuana laws, and they underscore the persistent uncertainty about how such conflicts are going to be resolved. Should the Federal Government arrest and prosecute marijuana users in States where they might be in full compliance with State law? Or should the Federal Government take a completely hands-off approach and let drug laws and policy develop on a State-by-State basis? Or is there some middle-ground approach that considers both the national interests and the fundamental principles of federalism, including the rights of voters to decide what is best for their own individual States? A total of 21 States have legalized marijuana for medical purposes, and 16 of those States have decriminalized the possession of small amounts of marijuana. But every one of these changes in State marijuana laws has taken place against the same background: the possession of any amount of marijuana is still a criminal penalty under federal law.