Understanding electronic cigarettes’ effect on tobacco smoking is a central economic and policyissue. This paper examines the causal impact of e-cigarette availability on conventional cigaretteuse by adolescents. First, synthetic control analyses consider how state bans on e-cigarette salesto minors influence teen smoking rates. These bans yield a statistically significant 1.0 percentagepoint increase in recent smoking in this age group, relative to states without such bans. Next, Iexamine survey data on cigarette and e-cigarette use, separating teens by estimated propensity tosmoke in the absence of e-cigarettes. Among those with the highest propensity to smoke, ecigaretteuse increased most while cigarette use declined: a 1.0 percentage point rise in ever useof e-cigarettes yields a 0.65 percentage point drop in this subgroup’s current smoking rate. Bothsets of results indicate a harm reducing effect of e-cigarettes on adolescent cigarette smoking, atleast prior to 2014.