The purpose of this explanatory research is to evaluate the impact of medical marijuana legalization (MML) in Colorado on crime. Specifically, does legalization lead to a decline in crime rates associated with marijuana (property crimes, burglary, larceny/theft, violent crimes, robbery, driving under the influence) in comparison to the national crime rate? This study used existing aggregated data (national crime rates, state of Colorado crime rates) and interrupted time series regression to examine the effect of MML on violent crimes, property crimes, and DUI for alcohol arrests. Overall findings indicate that MML significantly reduced two types of crime – burglary and robbery. For violent crime, property crimes, and DUI, the effect was neutral (neither increasing nor decreasing crime).