May a state authorise the use of marijuana for recreational purposes even if such use is forbidden by federal law? This novel and unresolved legal question has vexed judges, politicians, and legal scholars, and it has also generated considerable public debate among supporters and opponents of "legalising" the recreational use of marijuana. This book summarises the Washington and Colorado marijuana legalisation laws and evaluates whether, or the extent to which, they may be pre-empted by the Controlled Substances Act (CSA) or by international agreements. It also highlights potential responses to these recent legalisation initiatives by the U.S. Department of Justice (DOJ) and identifies other non-criminal consequences that marijuana users may face under federal law.