Cannabis remains the most commonly used illicit drug, and its use is on the rise among young adults. Changes in legal status, perceptions of low associated risk and ease of availability in social settings have been associated with increased rates of recreational cannabis use. With increased exposure comes the increased likelihood of detrimental impacts on psychological and behavioral outcomes. Previous laboratory research has found acute effects of cannabis use on impulse control and social functioning but very little is known about the real world effects of recreational and regular cannabis. It is also unknown whether momentary cannabis use effects relate to long-term detrimental changes associated with cannabis use, effects such as lowered quality of life, diminished psychosocial functioning, and increased or hazardous use of cannabis. The proposed study will address this knowledge gap by implementing ecological momentary assessment (EMA) to examine within- person responses to cannabis use in real-world contexts in order to understand what effects cannabis is having on the daily psychological and behavioral experiences of non-dependent recreational and regular cannabis users. This proposal will use intensive, repeated assessments delivered on smartphones to assess occasions of cannabis or other substance use (alcohol, tobacco, or other drugs), and within-person fluctuations in impulse control and social interactions in 120 recreational and 120 regular cannabis users (18-30 years). We hypothesize that (1) occasions of cannabis use will increase impulsivity, hostility, and risk behaviors and decrease positive social interactions relative to occasions where no use occurred. We also hypothesize that (2) frequency of use (recreational vs. regular) will moderate the acute response such that more frequent users will experience the greatest acute increases in psychological and behavioral effects. Using a novel approach, we will also examine (3) whether an individual's real-world responses to cannabis use can predict future changes in cannabis use, psychosocial functioning, and quality of life over the course of two years of follow-up. This study is innovativ in its method and approach to assessing pathways by which detrimental effects occur after cannabis use, how these pathways are enhanced by increasingly frequent cannabis use, and determine if effects within the same individual over time can be used to predict long term effects on psychosocial functioning and the onset of cannabis use disorders. This knowledge is vital to identifying the under-recognized impacts of cannabis use on psychological and social functioning. If our hypotheses are supported, we will identify how, and for whom, recreational and regular cannabis use leads to detrimental short term and long term functioning outcomes in young adults. Through this work, we hope to discover novel approaches for monitoring and determining future risk of hazardous cannabis use. These targets may then be used in developing innovative strategies for prevention and intervention of cannabis use in young adults; approaches that can be delivered via `just-in-time' mechanisms on smartphones.