Purpose. Smoking is the leading cause of death and smoking initiation rarely occurs after adolescence. Thus research on adolescent smoking is crucial. A consistent definition of smoking is important because inconsistent definitions make comparisons across studies an arduous task. Thus, the aim of the study was to suggest future research directions for smoking definitions after examining pitfalls in the literature. Methods. In this study the literature on adolescent smoking in the U.S. and South Korea was examined, and three types of smoking definitions were identified. Limitations in the studies are identified and future research directions are suggested. Results. In the U.S. literature, smoking definitions can be categorized into three groups: definitions based on stage models, smoking trajectories, and definitions derived from specific data. In the South Korean literature, various levels of smoking have not been differentiated. Conclusions. While the literature does not provide a definitive answer regarding the definition of smoking, it suggests three issues to consider for future research. First, multiple measures of smoking are more desirable than a one-time measure. Second, theory- or trajectory-based smoking definitions are more desirable than definitions derived from available data. Finally, regularity and amount of cigarette use should be incorporated in defining smoking behavior.