The scientific community has struggled to keep pace with the impact of the changing landscape of cannabis use and policy in the U.S. An important barrier to progress is the lack of adequate measures of cannabis consumption that can be used to identify low vs. high risk use patterns. The project, Leveraging Social Media to Develop the Cannabis Exposure Index (CEI), A Standardized Measure of Cannabis Use proposes to address this challenge via a rigorous cannabis measure development/validation project that will enhance the capacities of clinical, epidemiologic and policy studies and lead to more informed communication about cannabis and its risks to clinicians, health educators and policy makers amidst the rapidly changing landscape of cannabis use and policy in the U.S.