Participants in this research study have Type 2 diabetes mellitus (also called adult-onset diabetes, Type II Diabetes, or non-insulin dependent diabetes mellitus). Type 2 diabetes mellitus is the most common form of diabetes. It can lead to diseases of the eyes, nerves and kidneys, but heart disease is the major disease risk. Diet plays an important role in treatment of this kind of diabetes in several ways. The main goal of diet therapy is improvement in blood glucose levels. Glucose levels may be improved by the type of food eaten and also by weight loss. Fat levels in the blood (e.g. cholesterol and triglycerides) represent other important factors that can be affected by diet in diabetes. Generally, high fats in the blood are associated with increased risk of heart disease. Dietary advice for people with diabetes has changed over the past 30 years. Recently a diet high in carbohydrates with no more than 30% of calories from fat has been recommended. However, there has also been significant interest in using a diet higher in monounsaturated fats (special types of fat that you eat, like olive oil and canola oil) and lower in carbohydrates (fruits, vegetables and starches). Some studies have shown that a diet high "mono" (monounsaturated fats) results in lower sugars and improved fat levels in the blood. Several questions about the best diet for diabetes are still not clearly answered. The purpose of this study is to measure the glucose, insulin and lipid responses to a low fat, high carbohydrate diet compared to a high mono-unsaturated fat, low carbohydrate diet. In addition, the study will evaluate how each diet affects how much of the foods are eaten. We will also give nutrition questionnaires during each diet and also questionnaires which measure sense of well-being.