This chapter describes medical nutrition therapy for adults with diabetes and related topics. Medical nutrition therapy (MNT) provided by the registered dietitian leads to improved clinical outcomes and decreased costs associated with physician time, use of medication, and hospital admissions of individuals with obesity, diabetes, lipid disorders, and other chronic diseases. Medical nutrition therapy has long been considered the foundation or cornerstone of managing diabetes and many other nutrition-related conditions. This chapter will address the origins of MNT and its comparison to diabetes self-management training (DSMT) which can also be provided by the registered dietitian. This chapter covers the medical nutrition therapy, diabetes self-management training, and prevalence of diabetes, weight management and diabetes, nutrition and bariatric surgery, mediterranean, dietary approaches to stop hypertension, and low-carbohydrate diets, and gestational diabetes mellitus.