Substance use and criminal offending are associated with negative social and health consequences, both for individuals who engage in these behaviors and for those affected by them. It is well known that substance use and criminal offending often co-occur. Despite this, research on substance use and criminal offending has often treated the two as separate, which has resulted in gaps about how co-occurring behaviors develop, trajectories, and whether they are associated with worse outcomes than single behaviors. This dissertation therefore aimed to examine the development of co-occurring substance use and criminal offending in adolescence by examining different patterns, risk factors, trajectories, and outcomes. Two samples were used: A prospective longitudinal project, Futura01, of randomly selected individuals in Sweden born in 2001; and a compilation of Swedish register data on individuals who have received compulsory residential care at facilities operated by the Swedish National Board of Institutional Care during adolescence. Study 1 investigated if risk factors were more similar or unique, and more severe or accumulative for co-occurring alcohol misuse and criminal offending compared to alcohol misuse and criminal offending only. The results showed that risk factors were more similar than unique, and more accumulative than severe. Study 2 identified latent classes of co-occurring substance use and delinquency in adolescence and associated risk factors. The results suggested that co-occurring behaviors were uncommon in the general population, but those that engaged in co-occurring behaviors showed a multifactorial risk profile with many severe risk factors. Study 3 identified trajectories of co-occurring alcohol use and criminal offending and what negative social and health-related outcomes they are related to. The findings showed that co-occurring development was less common, as only a minority showed an upward trajectory of co-occurring alcohol use and criminal offending. This trajectory, despite being the least common, was associated with a larger number of negative consequences, suggesting that co-occurring behaviors are more detrimental than single behaviors. Study 4 examined long term social and health outcomes of compulsory residential care for co-occurring substance misuse and criminal offending in adolescence. The results indicated that those who received compulsory residential care for substance misuse only, and co-occurring behaviors, suffered the worst outcomes indicating that substance misuse was a clear driving factor in this population and that co-occurring behaviors resulted in more negative consequences than other behaviors. It also suggested that the closer to date of discharge, the greater vulnerability for negative outcomes, indicating that this is a period that might require additional support. The overall results of the dissertation suggest that extensive co-occurring behaviors are uncommon in the general population, but they do co-occur since one behavior was rarely present without the other. Similar risk factors also seemed to predict both substance use and criminal offending and those who engaged in co-occurring behaviors likely had a greater number of risk factors present. Despite being less common, co-occurring behaviors were associated with far more negative consequences. This may have implications for policies and interventions that aim to reduce the levels of substance use and criminal offending.