ABSTRACT Major changes in marijuana legislation are likely to affect not only the use of marijuana, but also the use of other substances including alcohol. Alcohol and marijuana are psychoactive substances commonly used by young adults in the US and are independently associated with numerous acute and long-term consequences. To date, recreational marijuana use has been legalized for adults age 21+ in four states (Washington, Colorado, Oregon, and Alaska) and the District of Columbia, with numerous states currently debating similar legislation. These policy changes have shifted attention to the important individual and public health implications of simultaneous alcohol and marijuana (SAM) use, and potential additive or synergistic negative effects and consequences (e.g., increased risk of driving while intoxicated, fatal car accidents). Among youth and adults, clear patterns are emerging; perceptions of the harmfulness of marijuana use are decreasing and the prevalence of marijuana use is increasing. Further, it is clear that SAM use is common among youth and young adults. Given the public health costs associated with alcohol use, there is a pressing need to understand the ways in which SAM use may have complementary effects (both alcohol and marijuana use increase) or substitution effects (amount of alcohol use decreases when used with marijuana), as well as the extent to which SAM use is associated with increases in acute and longer-term consequences of alcohol use. To date, very little is known regarding how SAM use differs from use of either substance alone on a given occasion. The current application proposes to collect and analyze intensive data collected twice daily via mobile phones for 16 weeks (using interactive voice response) across two years and longitudinal data collected annually online across three years from a high-risk community sample of young adult SAM users, aged 18-25, in the state of Washington. Specific aims of this application are to: 1) examine whether SAM use poses additional alcohol- related risks, compared to using alcohol without marijuana, at the daily level; 2) understand the motivations, intentions, and situations that make SAM use more likely and more consequential among young adults; 3) model the longitudinal patterns of SAM use and associations with health consequences across 3 years; and 4) examine potential moderators of these relationships. The proposed study will examine SAM use in a state where marijuana is legal and the observed effects will add knowledge about the effects of marijuana on alcohol use and related health consequences among young adults in the US.