Obesity is a major public health problem, and although short-term weight loss is achievable, individuals often regain the majority of weight that was lost. Current approaches have adjusted behavioral prescriptions and lengthened treatment contact in an effort to address this problem, with modest effect. This NIDDK funded R01 project is a randomized controlled clinical trial to compare the efficacy of an intervention based on Acceptance and Commitment Therapy (ACT) and a Self-Regulation (SR) intervention on weight loss maintenance over a 30-month period. All participants will first complete a well-validated online weight loss intervention (months 1-3 of the study). Participants who lose 4 kilograms of initial weight will then be randomly assigned to receive ACT or SR, with both conditions consisting of face-to-face, group-based intervention meetings and weekly email contact for 6 months. The ACT intervention will target a novel theoretically derived intervention target, values-consistent behavior, which will help align weight loss goals with personal values and foster internal motivation to continue with weight control efforts. Assessments will be at baseline, post-weight loss/pre-randomization, and then 6, 12, 18, 24, and 30-month follow-up (months from randomization). This project will be conducted at the Weight Control and Diabetes Research Center (WCDRC). The WCDRC is part of the Centers for Behavioral and Preventive Medicine at The Miriam Hospital in Providence, Rhode Island, and the Department of Psychiatry and Human Behavior at the Alpert Medical School of Brown University. This study is significant because it addresses weight loss maintenance - a critical barrier to treating obesity - and results of the study could be used to improve long-term weight loss outcomes and associated health benefits for treatment seeing overweight and obese adults.