Objective: Very few studies have examined the developmental nature of comorbid alcohol use disorders and drug use disorders (AUD-DUD). The current study sought to extend our understanding of the nature of AUD-DUD comorbidity by characterizing the developmental course of AUD-DUD comorbidity, determining the degree to which the two disorders occur during the same period, and eliciting differences in AUD-DUD trajectories over the life course. Method: Vietnam-era male veterans and matched civilians provided retrospective accounts of alcohol- and other drug–related experiences spanning 25 years. Concurrent growth mixture modeling was used to describe the course of AUDDUD lifetime comorbidity. Results: Five trajectories were identified based on the probabilities of diagnosing with AUD-DUD: substance switching (increasing AUD, decreasing DUD); young adult (both AUD and DUD decreasing rapidly after young adulthood); severe nonchronic (both AUD and DUD decreasing slowly in the third decade of life); severe chronic alcohol–severe nonchronic drug (AUD remains high and DUD decreases in the fourth decade); and young adult alcohol–severe chronic drug (decreasing AUD in the second decade and DUD remains high). Conclusions: For the majority of this sample, substance use disorders continued or worsened through adulthood, indicating a problem that extends far beyond young adulthood. Demographic characteristics differed among the trajectories; however, psychiatric diagnoses did not differ except for the number of years with diagnoses of antisocial personality disorder. Subthreshold symptoms in adulthood may be present for a significant period before diagnosis, making this period important for intervention and prevention. Integration of efforts into healthcare, employment, and public policy will help target those at highest risk.