Unhealthy eating patterns and weight gain have been reported in people recovering from substance addiction. People in recovery are at an increased risk for diet-related chronic diseases, and environments in drug treatment facilities may be unsupportive of healthy food choice behaviors. Recovery Healthy Eating and Active Learning in Treatment Houses (RHEALTH), a theory-based pilot nutrition intervention was designed to promote healthy eating and reduce excess weight gain among men in residential treatment programs through both food and nutrition education classes and changes in the food environment in the treatment facilities via the promotion of healthy food-related policies. This dissertation reports the outcome and process evaluation of the implementation of RHEALTH in six residential treatment facilities for men in recovery from drug addiction. The study participants were 124 men (103 men enrolled before the control and 21 men enrolled after the control period ended), aged 18 years and older in recovery from substance addiction, and who were assessed at three different times: at baseline, at pre-intervention following a six week control period, and at post-intervention. The main intervention outcomes were dietary intake, diet-related psychosocial factors, measured height and weight from which body mass index was derived, and waist circumference. Independent variables included demographic characteristics, selfreported addiction history and physical activity. A structured process evaluation assessed the impact of program participation, dose, fidelity and intervention implementation levels on study outcomes. At baseline study participants reported poor dietary intakes; average intakes exceeded recommendations for total energy, fats and sweets but were inadequate in daily servings of fruits and vegetables. Seventy-three percent of the men were either overweight or obese, and 36% had a waist circumference that put them at risk for chronic disease. Multivariate analysis at baseline also revealed that better dietary quality was associated with longer time spent in the treatment program, higher selfefficacy for healthy eating among younger participants, readiness for diet change in the next 30 days, and satisfaction with diet and weight. Lower body mass index and waist circumference were associated with younger age, higher educational levels, and with higher self-efficacy among men who were satisfied with their diet and weight. The outcome analysis included 55 men who had completed two or more assessments (43 who completed all three assessments and 12 who completed only the baseline and post-intervention). Mixed model regression analysis was used to assess the impact of the RHEALTH intervention on dietary intake, diet-related psychosocial factors, and body composition measures. In multivariate analyses, the intervention period was associated with significantly greater intake of fruits and vegetables, lower intakes of calories from sweets and desserts, and greater improvement in food-related psychosocial factors compared to the control period. Reduction in waist circumference was also associated with the intervention period. The process evaluation revealed that better study outcomes were associated with both greater participation in food and nutrition classes and a higher level of implementation of healthy changes in the food environment. Challenges to implementation included staff and resident turnover and staff commitment. The findings of this pilot intervention provide evidence supporting the need for and the potential for efficacy of large-scale randomized dietary intervention trials aimed at increasing healthy food choice behaviors and healthy food environments in drug treatment facilities.