Parental monitoring serves as a key element in the reduction of adolescent substance use, but little is known about how various monitoring practices relate to different levels of use. This study examined the association between two forms of parental monitoring (parental knowledge and parental supervision) and two substance use outcomes (alcohol and marijuana) for Hispanic and non-Hispanic White eighth-graders using the Arizona Youth Survey (n = 11,703). Results demonstrated that parental supervision and parental knowledge protected against alcohol and marijuana use for both ethnic groups, but tests for ethnic differences indicated that parental supervision was more protective against high levels of marijuana use for White youth compared to Hispanic youth. Gender differences were also observed with girls benefitting more from both forms of monitoring compared to boys. However, these effects were dependent on the level of substance use involvement and the substance use outcomes. (Published Abstract)