ABSTRACTBackground: Alcohol and marijuana are the most commonly used substances among adolescents but little is known about patterns of co-use. Objectives: This study examined patterns of concurrent (not overlapping) and simultaneous (overlapping) use of alcohol and marijuana among adolescents. Methods: Data from US-national samples of 12th graders (N = 84,805, 48.4% female) who participated in the Monitoring the Future study from 1976 to 2016 and who used alcohol and/or marijuana in the past 12 months were used to identify latent classes of alcohol use, marijuana use, and simultaneous alcohol and marijuana (SAM) use. Results: A four-class solution indicated four patterns of use among adolescents: (1) Simultaneous alcohol and marijuana (SAM) use with binge drinking and recent marijuana use (SAM-Heavier Use; 11.2%); (2) SAM use without binge drinking and with recent marijuana use (SAM-Lighter Use; 21.6%); (3) Marijuana use and alcohol use but no SAM use (Concurrent Use; 10.7%); and (4) Alcohol use but no marijuana or SAM use (Alcohol-Only Use; 56.4%). Membership in either SAM use class was associated with a higher likelihood of truancy, evenings out, and use of illicit drugs other than marijuana. SAM-Heavier Use, compared to SAM-Lighter Use, class members were more likely to report these behaviors and be male, and less likely to have college plans. Conclusions: Among 12th graders who use both alcohol and marijuana, the majority use simultaneously, although not all use heavily. Given the recognized increased public health risks associated with simultaneous use, adolescent prevention programming should include focus on particular risks of simultaneous use.