Type 2 diabetes mellitus (T2DM) is a common metabolic condition influenced by impaired insulin secretion and insulin resistance. The Dietary Insulin Index (DII), a measure of food-induced insulin responses, has gained attention for assessing T2DM risk. Unlike the glycemic index, DII considers insulin responses from both carbohydrates and proteins, offering a broader perspective on dietary effects. This chapter discusses how high DII is linked to T2DM through pancreatic β-cell overload, insulin resistance, and metabolic issues tied to obesity and inflammation. High-DII diets may also worsen oxidative stress, promote fat storage, and lead to visceral obesity, heightening diabetes risk. Compared to glycemic indices, DII may better predict insulin response, especially for foods that elevate insulin without increasing blood glucose. Low-DII foods support improved insulin sensitivity, reduced inflammation, and better glucose control, highlighting their potential role in T2DM.