Obesity is an epidemic in the United States, and it is extremely costly to care for those with obesity related illnesses. Behavioral lifestyle interventios have been shown to successfully produce clinically significant weight losses and improve health. Internet- delivered weight control programs have been shown effective in producing clinically- significant weight losses, with the approach to web-based obesity treatment our research group has developed producing some of the largest weight losses reported in the literature to date. However, weight losses remain modest with online delivery, undercutting the potential impact of technology-enhanced delivery. Therefore, the overall goal of this project is to continue the work of our group in identifying strategies to enhance weight loss outcomes produced by an online-delivered behavioral program by examining the promising strategy of providing financial incentives within the context of treatment. We propose a randomized controlled trial to determine whether incorporating financial incentives increases the amount and duration of weight losses achieved by Internet-delivered behavioral treatment. A secondary, exploratory aim is to quantify the incremental cost-effectiveness of each intervention strategy compared to established cost-efficacy thresholds. Overweight and obese adults at two study sites (N=416; 27% minority) will randomized to: (1) Internet intervention; or 2) Internet intervention plus financial incentives for implementation of key self-management behaviors (daily self-weighing, self-monitoring of dietary intake and achieving step goals) (Internet+Incentives). All participants will receive the same 36- session web-based group weight control program which features synchronous, facilitated chats and online behavioral tools. Assessments will be conducted at 2, 6, 12- and 18 months and will include measures of body weight, treatment engagement (e.g., attendance, self-monitoring, website utilization, motivational factors, weight control behaviors) and treatment delivery cost.