Opposing theories posited that young adult substance use during the COVID-19 pandemic would decline due to restricted access and social engagement or increase due to efforts to cope with rising distress, loneliness, and isolation. Peer-reviewed global longitudinal studies found (a) overall declines in commonly used substances in 2020 with some rebounding in 2021; (b) individual differences in this pattern, with declines linked to factors limiting access and increases to pre-existing risk; and (c) under-developed evidence to evaluate increased coping-related use. Given potential links between surging mental health disorders and substance use, further surveillance and wider dissemination of substance use programming is needed, particularly for vulnerable individuals and settings.