This study investigated relationships between local alcohol policies, enforcement, alcohol outlet density, adult alcohol use, and underage drinking in 50 California cities. Method: Eight local alcohol policies (e.g., conditional use permit, social host ordinance, window/billboard advertising) were rated for each city based on their comprehensiveness. Local alcohol enforcement was based on grants received from the California Alcoholic Beverage Control agency for enforcement of underage drinking laws. Outlet density was based on the number of on- and off-premise outlets per roadway mile. Level of adult alcohol use was ascertained from a survey of 8,553 adults and underage drinking (frequency of past-year alcohol use and heavy drinking) from surveys of 1,312 adolescents in 2009 and 2010. Multilevel regression analyses were conducted to examine the effects of policies, enforcement, and other community-level variables on adolescent drinking, controlling for youth demographic characteristics. Mediating effects of adolescents' perceived ease of obtaining alcohol, perceived enforcement, and perceived acceptability of alcohol use also were examined. Results: None of the eight local alcohol-policy ratings were associated with adolescent drinking. Funding for underage drinking enforcement activities was inversely related to frequency of past-year alcohol use, whereas outlet density and adult drinking were positively related to both past-year alcohol use and heavy drinking. These relationships were attenuated when controlling for perceived ease of obtaining alcohol, enforcement, and acceptability of alcohol use, providing evidence for mediation. Conclusions: Adolescent alcohol use and heavy drinking appear to be influenced by enforcement of underage drinking laws, alcohol outlet density, and adult alcohol use. These community-level influences may be at least partially mediated through adolescents' perceptions of alcohol availability, acceptability of alcohol use, and perceived likelihood of getting in trouble with local police.