ABSTRACT The 2016 legalization of recreational cannabis in California created a legal growth, distribution, sales, and taxation system, which is still going through an implementation process dependent on local policy options. Presently, there is considerable local variation in cannabis policies, along with legal retail availability and illegal trafficking throughout the state. This 5-year component of the Center will leverage these unique varying community contexts to examine the associations between local alcohol and cannabis control policies, alcohol and cannabis co-use and related problems among non-Hispanic Whites and Hispanics. In 2018, these two groups together comprised 76% of the population in California. The study will be the first to investigate how the local policy and retail environment (and the illegal cannabis market) contribute to or possibly ameliorate ethnic disparities in alcohol and cannabis use and related harms with longitudinal analyses of survey data. Existing evidence indicates overlap in the use of these two substances. For instance, 64% of adults who used cannabis in the past year, used alcohol in the past month or used alcohol heavily in the past month. Co-use of these two substances, especially simultaneous use, has been associated with a higher rate of personal and social problems. Concurrent users are 2 times more likely than alcohol-only users to report social problems from use, while simultaneous users are almost 3 times more likely than alcohol-only users to report problems and harms to self. Survey data will be collected over the phone and online from 18-39 year old Whites and Hispanics in years 1, 2, and 3 (T1, T2, T3) of the project (N=1,200) in 40 cities that vary in alcohol and cannabis policies and retail availability. The specific aims are: 1) examine the associations between local regulatory policies, enforcement activities, proximity, and daily exposure to licensed alcohol and cannabis outlets, and illegal cannabis sources; 2) examine the associations between increased exposure to both legal (alcohol and cannabis outlets) and illegal cannabis markets and patterns of use, co-use, and associated problems; 3) examine contexts of alcohol and cannabis use and co-use, variation in types of cannabis use (e.g., smoke, vape, edible) and co-use by contexts, and the association of contexts and types of use and problems; 4) investigate T1 to T3 changes in local policy, policy enforcement and alcohol and cannabis retail availability and T1 to T3 changes (increase, decrease, incidence, stability, cessation) in alcohol and cannabis co-use.