Self-weighing is a simple and cost-efficient weight management strategy. However, not everyone can benefit from it. Psychiatrists and dietitians are concerned about recommending it to their patients. The broad objective of this dissertation was to examine the effect of self-weighing on weight outcomes and behaviors, and identify the best recipients for the daily self-weighing intervention. In a two-year, randomized-controlled, daily self-weighing study among workplace employees (Weigh2Go study), I examined the effect of daily self-weighing to prevent age-related weight gain, identified participants’ baseline characteristics that predicted successful weight outcomes using a mixed-methods approach, and explored the relationship between early weight variability and the following weight gain. To explore the effect of self-weighing on behaviors, I conducted a four-week experiment among college students on their food purchasing behavior (Trillium study). In the Weigh2Go study, the classic intent-to-treat analysis revealed that the daily self-weighing intervention group did not differ significantly from the matched control group in two-year weight changes. Further exploratory analyses revealed that, when only the participants that were compliant with their group assignment were included, the intervention group achieved better weight gain prevention outcomes than the control group. Additionally, participants who benefited most from daily self-weighing had unique characteristics at baseline (e.g., less depressed, fewer eating disorder symptoms, less body image dissatisfaction, more mindful). From the interviews, participants with an initial negative perception of daily self-weighing before the study were less likely to maintain or lose weight at the end of the study. Analysis of the variability in body weight revealed that early weight variability was not associated with the subsequent weight gain and thus did not affect the weight outcomes of the Weigh2Go study. Finally, the Trillium study showed self-weighing just before entering a dining hall does not affect the food purchasing behavior among college students, suggesting self-weighing might not affect meal intake as a mechanism to prevent weight gain. Overall, this dissertation examined the effects and predictors of success of using self-weighing to prevent age-related weight gain. It also pointed out the problems involved in being able to demonstrate the effectiveness of treatments that may prevent age-related weight gain.