The overarching goal of our proposed project is to develop an efficacious standalone text messaging obesity intervention. We focus on standalone approaches -- treatments that can be wholly delivered via text messaging -- because of their dissemination potential. Standalone treatments are readily scalable, can be used modularly, and several such interventions (e.g., Text4baby) have achieved population reach. Despite their translational potential, no trials have yet tested a fully standalone texting intervention for weight loss. To accomplish this goal, we propose to use the multiphase optimization strategy (MOST) to identify which intervention components should optimally be included in our standalone texting weight loss intervention. MOST guides the randomized experimentation of intervention components so that we might assemble a treatment package comprised only of components that make a meaningful impact on weight loss. We will conduct a 12-month experimental trial among 592 obese adults. All study participants will receive a core 6-month weight loss texting intervention (based on our iOTA approach) that includes tailored behavior change goals, interactive self-monitoring, automated feedback, and skills training. Using a fractional factorial design, we will randomize participants to one of 16 experimental conditions (37 in each condition) that will test the text messaging components (and levels). We will follow participants at months 1, 3, 6, and 12. We will experimentally determine which text messaging components (and component levels) produce a meaningful contribution to 6- month weight change and 6-month change in diet, physical activity, and the proportion of participants who achieve > 5% weight loss at 6 months and maintenance at 12 months. We will examine associations of engagement and non-usage attrition with weight change, model trajectories of change in engagement and non- usage attrition over time, and test theoretically-driven mediators of engagement, self-efficacy, and the Technology Acceptance Model constructs: perceived ease of use and perceived usefulness. We will also determine the cost of the intervention’s implementation and its incremental cost effectiveness. Finally, we explore whether combinations of intervention component are differentially efficacious by participant characteristics; this will allow us to ultimately tailor intervention structures for optimal outcomes. This study will demonstrate how to best assemble a low cost, high reach standalone texting weight loss intervention.