Inclusion Criteria:  Enrolled in grade 9-12 at participating high school Current e-cigarette user, as defined as a response greater than "0 days" to the question: "During the past 7 days, on how many days did you use e-cigarettes?" This eligibility criterion is modeled on a question in the 2018 National Youth Tobacco Survey (NYTS), a survey of U.S. middle and high school students which used a 30-day timeframe. Have a smartphone. English- or Spanish-speaking  Exclusion Criteria: Unable or unwilling to provide informed assent or consent (in the case of those aged 18 years or older).