The legalization of recreational marijuana in Alaska, Colorado, Oregon, and Washington State is the most dramatic change in U.S. substance abuse policy since the end of Prohibition. These states are implementing regulatory and enforcement systems for the retail sales of recreational marijuana to adults 21 years or older, akin to state controls on the sale of alcohol. In 2013, the U.S. Department of Justice (DOJ) issued directives that it would monitor whether these states implemented strong and effective regulatory and enforcement systems that among other DOJ priorities, prevented distribution of marijuana to youth. A retail sales intervention that has been effective at reducing illegal alcohol sales to minors (and intoxicated patrons) is alcoholic beverage server training (or responsible beverage service training). Recently, Colorado enacted a law for incentivized responsible marijuana vendor training, Oregon will require responsible marijuana vendor training, and Washington State may incentivize it. In this study, we propose to 1) produce the TrainToTend online responsible marijuana vendor training presenting retail marijuana sales laws, methods for checking IDs and preventing third party sales, the health effects of marijuana (e.g., fetal exposure; safe storage), customer service (e.g., driving under the influence; refusing sales to minors), and rules of the trade (e.g., inventory tracking and safety procedures) and 2) conduct a randomized trial on the TrainToTend training with a sample of retail recreational marijuana stores in Colorado, Oregon and Washington State (n=150) that are randomly assigned to have their employees complete the TrainToTend training (experimental group) or receive usual and customary sales training by store managers (no intervention control group). The effect of the TrainToTend training on responsible sales practices will be assessed by changes in recreational marijuana purchase attempts by pseudo-underage patrons (i.e., young-appearing customers without apparently valid IDs) at each store measured at 3-month follow-up and 9-month follow-up relative to baseline.