Background: Low-cost health interventions are needed to prevent disease at the population level. Advances in technology offer researchers opportunities to test new strategies and modalities. This dissertation examined the utility of a wearable device/sensor with website (Fitbit One) and SMS text messaging prompts to increase physical activity (PA) in overweight/obese adults. It also examined participants' level of engagement with intervention components and factors associated with engagement. Methods: In a 6-week randomized controlled pilot trial, 67 participants were provided a Fitbit One for self-monitoring and half were randomized to receive daily text messaging PA prompts. Primary outcomes were objectively measured steps and minutes of PA by intensity level using Actigraph GT3X+ and Fitbit One. Self-reported baseline and follow-up surveys were analyzed to assess participants' attitudes and level of engagement with intervention components (Fitbit One and text messages), and whether self-efficacy and outcome expectations could predict level of engagement. Secondary data were analyzed from the Women's Healthy Eating and Living (WHEL) Study to examine adherence to study protocols and dietary change by baseline depressive symptoms in women who were enrolled in a large dietary trial that focused on behavioral activation components. Results: (1) Daily text messaging prompts were associated with short-term PA increases but only for 1 week, and Fitbit One (without text messages) was associated with an increase in MVPA at week 6. (2) Participants who used the Fitbit mobile app increased their steps at week 6. (3) Baseline depressive symptoms predicted lower adherence to study protocols, but only in control participants, and (among those furthest from recommended targets at baseline) less improvements in change in plasma carotenoid concentrations, fiber, and fat. Conclusions: Participants' level of engagement with the study intervention was associated with health behavior change. Text messages as PA prompts were insufficient to keep participants engaged to maintain PA changes beyond 1 week. Fitbit One, particularly the Fitbit mobile app, which is both accessible and provides instant detailed feedback for self-monitoring, was helpful in increasing PA. Lastly, a dietary intervention with behavioral activation components increased level of engagement in both non-depressed and depressed women who made dietary improvements.