Medical nutrition therapy (MNT) is a cornerstone of treatment for the estimated 20.8 million people with diabetes in the United States. MNT is a more intensive and focused comprehensive nutrition therapy service that relies heavily on follow-up and provides repeated reinforcement to help change behavior. The long-term goal of medical nutrition therapy in diabetes is to prevent and/or delay diabetes complications by restoring metabolism to as close-to-normal as possible. Strategies used in MNT differ depending on the type of diabetes. In type 1 diabetes, the focus may be coordinating insulin treatment to diet and physical activity, and in type 2 diabetes, the focus may be weight reduction. This chapter will review the goals of MNT in type 1, type 2, and gestational diabetes. We will review macronutrient composition including carbohydrate metabolism, micronutrient composition, and vitamin use in diabetes. We will clarify the terms used to describe carbohydrates and how they affect blood glucose (glycemic index, glycemic load, advanced glycosylation products, net carbohydrate, available carbohydrate, and glycemic glucose equivalent). Additionally, strategies for decreasing energy intake (lowering dietary energy density, reduced portion size, meal replacements, and structured meal plans) will be discussed. MNT is an integral component of diabetes prevention, management, and self-management education. All care providers involved in diabetes treatment need to be knowledgeable about nutrition therapy to help individuals with diabetes achieve recommendations for a healthy lifestyle.