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 21+ in four states (Washington, Colorado, Oregon, 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 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 (use of alcohol decreases when used with marijuana), as well as the extent to which SAM use is associated with increases in acute and long-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 is to collect and analyze intensive daily (twice daily for 16 weeks across two years) and longitudinal (annually across three years) data 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 more likely and more consequential among young adults; 3) model the longer-term associations between SAM use and substance use and health consequences across 3 years; and 4) examine potential moderators of these relationships. The proposed study will examine SAM use among a high-risk community sample in Washington State, where marijuana is legal and the observed effects will be leading indicators of the effects of marijuana on alcohol use and related health consequences among young adults in the US.