Despite the large body of research that has identified the impact of diet on cancer risk, little has been done to translate these findings to clinical settings. Here we address prevention of colorectal cancer (CRC), the third most prevalent cancer in the U.S. CRC is among the cancers that are most strongly affected by diet and excess adiposity, and incidence is increasing in young people below the age of 40. More efforts need to be directed at providing individuals with the tools needed for achieving and maintaining a preventive diet. Recommendations for cancer prevention from the American Cancer Society and American Institute for Cancer Research include advice to maintain a healthy weight, to consume more plant-based foods, and to limit red meats, processed meats, and foods with added sugar. This proposal addresses the development of methods that could feasibly be implemented in medical settings to assist high risk individuals in attaining both the weight management and diet quality goals for prevention of CRC. Building on our previous trials, this proposal seeks to test two dietary interventions in overweight and obese persons who have a strong family history of CRC or a personal history of CRC. We will recruit 240 participants who will be randomized to receive to 12 months of: 1) a control group that receives written information on preventive diets; 2) a Western Limit Diet that simply encourages limiting foods associated with increased risk of CRC: ultra-processed foods (foods containing refined flour, added sugars, or processed meats) and red meats; or 3) a Comprehensive Diet that combines goals for both limiting Western foods and consuming preventive foods. The two interventions will be supported by written educational materials, brief telephone contacts by case managers, and a different module of the MyGI app for each diet. MyGI incorporates checklists to facilitate real-time goal tracking, tailored messaging, and real-time display of progress made. The dietary changes are expected to improve diet quality and achieve a sustainable, slow rate of weight loss over time that appears to be more favorable for cancer prevention than conventional obesity treatment. The specific aims are: 1) To determine if an intervention limiting Western foods has similar effects as a Comprehensive intervention, versus Control, on the dual primary outcomes of improving diet and achieving modest weight loss in overweight and obese subjects at increased risk of CRC. Secondary outcomes include biological and behavioral measures of intervention adherence, app usability, and serum markers of inflammatory stress. 2) To evaluate whether skin carotenoids (as a biomarker of fruit and vegetables consumption) and breath ketones (as a biomarker of fat oxidation) can be utilized as non-invasive, objective measures of dietary compliance. The long-term goal is to identify an intervention that could be incorporated as part of usual health maintenance care to help high risk patients meet and maintain dietary cancer prevention goals.