Nonmedical (recreational) cannabis use is now legal in many states. Additional states are considering legalization, despite the weakness of the current evidence base regarding potential impacts on public health. This R01 proposal seeks 5 years of funding to understand: Aim 1) changes in cannabis use across the first decade following cannabis legalization; Aim 2) changes in alcohol use, nicotine use, and their co-use with cannabis following cannabis legalization; and Aim 3) whether psychosocial consequences of cannabis use change in a legal cannabis context. This knowledge is critical to inform policy, support efforts to maintain hard- won reductions in teen cannabis use since the 2000s, and promote responsible use by adults. The proposed project is grounded in life course theories of development and substance use, and uniquely suited to address stated goals. It continues and expands upon the Seattle Social Development Project-The Intergenerational Project (SSDP-TIP), which aims to understand the impact of cannabis legalization on youth and parent cannabis and other drug use (n = 426 families, 80% living in Washington State). SSDP-TIP includes a parent drawn from another ongoing longitudinal panel study that began in 1985, their oldest biological child, and a second caregiver (usually the spouse) when available. Seven waves of pre-legalization data (2002-2011; mean child age in 2011 = 12) and three waves of short-term post-legalization data (2015-2017; mean child age in 2017 = 18) from offspring and parents are available. The proposed study will add three additional annual data collections post-legalization in 2022, 2023, and 2024 with parents and offspring (mean offspring age in 2024 = 25; range: 11-36 years). For adolescent offspring, the accelerated longitudinal design allows us to compare offspring measured before legalization to offspring from later birth cohorts measured at the same ages, but after legalization. This permits the disentangling of child age and p