The proposed research will provide in-depth, longitudinal data, based on real-time reports, which will address key priorities for the FDA including, increased understanding of the relations between electronic cigarette (e-cig) use and 1) nicotine dependence, 2) reductions in combustible cigarette (CC) use, 3) attempts to quit CC use and the success of those attempts, and 4) health-related outcomes such as biomarkers of exposure and carcinogenicity.