Heavy marijuana (MJ) use in adolescence has been associated with lowered IQ and cognitive impairment in later life. It is unclear if lighter and more sporadic MJ exposure (i.e. recreational MJ use), a pattern more typical of most adolescent users, has similar deleterious effects. In a recently completed 3-year longitudinal study, we found adolescents with recreational MJ use failed to show age-expected maturation in sustained attention, visuospatial working memory and executive functioning. The aim of this ALSPAC proposal is to replicate our study findings. Convergent results from the ALSPAC Study will greatly enhance the impact of these findings on public health policies, especially in view of adolescents’ perception of diminished harm from MJ and the current surge in the United States and around the world to liberalize laws governing medical and recreational MJ use.