There is intense debate over change in cannabis policy, and substantial actual change happening in many states. The debate largely focuses on the choice between “prohibition” and“legalization,” with “medical marijuana” and “decriminalization” understood as partial moves along the spectrum toward “legalization.” The actual range of choice is far wider and involves many dimensions, not just one.Much of the debate concerns abstractions and principles: liberty and consumer choice on the one hand, morality, child welfare, and health on the other. Assertions are made about concrete outcomes such as changes in arrest and incarceration, the size of illicit markets, non-drug crime, the extent of substance use disorder involving cannabis, problem use of other drugs, rates of cannabis use among minors and their impacts on educational attainment, driving under the influence, and so on, but the evidence base underlying those assertions is thin.