During the past several decades, smoking prevalence among youth has fluctuated in puzzling and unexpected ways. To help understand these changes, this study tests seven explanations: (a) compositional changes, (b) sample selection, (c) adult smoking, (d) social strain, (e) cigarette prices, (f) tobacco advertising, and (g) other drug use. Figures on smoking prevalence come from the Monitoring the Future (MTF) Surveys from 1976-2002, whereas figures on aggregate determinants for the same time period come from government publications. Graphs of the time-series trends to determine temporal correspondence and time-series regression models to test for statistical influence reveal two variables that have expected effects. Increases in cigarette prices reduce smoking, particularly in the most recent years, and higher marijuana initiation (or use) is associated with greater smoking during most of the time period. However, much of the change in youth smoking, particularly the most recent rise and fall, remains unexplained. (Contains 4 figures, 1 table, and 7 notes.)