 This study aimed to identify distinct groups of smartphone app login trajectories over a six-month period, their association with smoking cessation outcomes at 12 months, and baseline user characteristics that predict data-driven trajectory group membership in the context of a randomized trial comparing two smartphone apps for smoking cessation. The results showed that patterns of one dash, three dash, and four-week smartphone app use for smoking cessation may be common in how people engage in digital health interventions, with significantly higher odds of quitting smoking among four-week users and especially among 26-week users of the icon quit app. The study suggests that strategies for detecting users who disengage early from these interventions, one-week users, and proactively offering them a more intensive intervention could be fruitful to improve study outcomes. This article was authored by Jonathan B. Bricker, Kristen E. Moll, Margarita Santiago Torres, and others.