Fewer teenagers are participating in the labor force today than at any point since WWII. At just under 44 percent teen labor force participation is 15 percentage points below its peak in the late 1970s. The authors investigate the long-run decline in the work activity of young adults, and the acceleration of this trend during the last five years. They conclude that time will tell whether the large drop in teen labor force participation of recent years is a temporary product of a labor market that is weaker than the unemployment rate would suggest, or a more permanent drop driven by the increased attractiveness of study, changes in college financial aid, or something else. Available evidence suggests that the decline in participation is being accompanied by increases in human capital investments that may add to the future productivity of today's teenagers. (Contains 4 notes and 2 figures.)