PROJECT ABSTRACT Electronic cigarettes (EC) remain the most commonly used product among high schoolers. Little is known about acute and longitudinal respiratory effects of ECs. This novel study will contribute critical knowledge needed to understand pulmonary functioning and respiratory effects among 150 youth who are exclusive-EC (N = 100) or never-users (N = 50; EC or other tobacco products), using a longitudinal, multimethod design. Youth will complete 5-laboratory assessments to provide a comprehensive assessment of respiratory health (airway reactivity and inflammation; pulmonary functioning). Further, we will incorporate EC user topography to assess the immediate acute effects of EC use on respiratory markers. Two weeks prior to laboratory assessments youth will complete daily EMA and spirometry assessments to examine how the frequency and quantity of EC use impairs acute pulmonary functioning. This Tobacco Regulatory K01 will provide much needed evidence regarding the immediate and longitudinal effects of EC use, while providing valuable training to a promising new investigator from leading experts in the field of tobacco regulatory science.