Abstract: "An increase in electronic cigarette (EC) use among US youth has raised concerns that their use may increase conventional cigarette smoking...Repeated cross-sectional data from two large nationally representative samples of US youth were drawn from the National Youth Tobacco Survey (NYTS) and Monitoring the Future (MTF). Students from grade 6–12 of the NYTS (N = 98,454–132,003) and students from grade 12 of the MTF (N = 17,870–30,981) were included in the analyses. Trends in smoking behavior, intention to smoke in the future and perceived smoking harms among youth who do and do not use ECs were estimated from 2014 to 2020 using weighted multinomial logistic regression and predicted marginal probabilities...The prevalence for regular smoking decreased among youth who regularly use ECs. For example, among regular EC users, smoking prevalence decreased from 27.8% to 6.7% (−21%; 99.67% CI:[−32.3%, −9.9%]) in the NYTS between 2014 and 2020 and from 31.8% to 10.6% (−21.2%, 99.67% CI:[−35.0%, −7.3%]) in the MTF between 2014 and 2018. Intent to smoke in the future and perceiving smoking as harmless decreased or remained unchanged during the same period. For example, among regular EC users, there was no significant change in intention to smoke next year in the NYTS between 2014 and 2018; intention to smoke in the next 5 years decreased significantly from 30.7% to 11.2% (−19.5%, 99.67% CI:[−37.7%, −1.3%]) in the MTF...In two national samples of US youth, smoking prevalence declined by a sizeable relative percentage. Intent to smoke in the future and harm perceptions of smoking declined or remained unchanged while EC use increased. Results provide little evidence that EC use has increased conventional cigarette smoking among youth".