ABSTRACT In recent years, cigarette smoking prevalence in the U.S. has declined slowly and young adults continue to have among the highest rates of smoking. Moreover, the market for emerging tobacco products is growing and has received a lot of media attention. Yet very little is known about the nature of using emerging products, traditional tobacco products, and poly-tobacco use among young adults. Current surveillance systems to monitor and examine the trajectories of tobacco use in young adults are undermined by inadequate sampling and data collection methodologies for reaching this important but difficult to reach population. Research from the current funding period demonstrated that cellphone random digit dialing (RDD) is an effective and innovative sampling methodology to reach a representative sample of young adults in the US. However, longitudinal data in this population is still sorely needed, but remains challenging given the technological environment in which to conduct surveys. Therefore, this competing renewal application seeks to develop and compare three data collection modes to longitudinally survey young adults that leverage cellphone RDD and/or smartphone technology in different ways. Specifically, we aim to 1) develop and implement an innovative mixed mode design to follow young adults longitudinally using cellphone RDD sampling and mobile data collection technologies, 2) assess the strength and limitations of each mode along data quality indicators for longitudinal studies via a randomized experiment, and 3) use collected longitudinal data to explore young adults' tobacco use trajectories during a time of rapid change. In addition to its potential to improve our understanding of tobacco use in young adults and support effective tobacco control regulatory policy, this research may provide real-world solutions for improving the collection of longitudinal data as well as more far reaching implications for survey research more generally.