An increasing number of states have adopted laws that legalise marijuana for medical or recreational purposes under state law, yet federal penalties remain. In 2012, Colorado and Washington became the first states to legalise marijuana for recreational purposes. In 2013, the Department of Justice (DOJ) updated its marijuana enforcement policy by issuing guidance clarifying federal marijuana enforcement priorities and stating that DOJ may challenge those state marijuana legalisation systems that threaten these priorities. This book examines, among other issues, DOJ’s efforts to monitor the effects of state marijuana legalisation relative to DOJ’s 2013 guidance; and factors DOJ field officials reported affecting their marijuana enforcement in selected states with medical marijuana laws.