The Look AHEAD trial, an acronym taken from the title “Action for Health in Diabetes”, has been designed to examine the long-term effects of a moderate amount of weight loss and increased physical activity on the incidence of serious cardiovascular events in overweight and obese individuals with type 2 diabetes. Weight loss will be achieved by an intensive lifestyle intervention program, which in many respects is adapted from that used in the recently completed Diabetes Prevention Program. The health effects of this program will be compared to a program of diabetes support and education, and participants will be randomized to either arm. Other outcomes, including cardiovascular disease risk factors, diabetes related metabolic factors and complications, and the cost-effectiveness of the intensive intervention will also be studied, and as well, there are a number of sub-studies and ancillaries studies examining factors such as sleep apnea, body composition, insulin sensitivity and other outcomes. The impetus for the Look AHEAD trial stems from a need to better understand the long-term health effects of weight loss. Approximately 5,000 volunteers with type 2 diabetes who are 45-75 years of age and overweight or obese (body mass index > 25 kg/m2) will be recruited. The primary outcome is the aggregate occurrence of severe cardiovascular events including fatal and non-fatal myocardial infarctions and strokes and cardiovascular deaths over a planned follow-up period of up to 11.5 years. A composite secondary outcome has also been defined, which consists of all deaths, myocardial infarction, stroke, coronary artery bypass grafting, percutaneous coronary angioplasty, hospitalization for congestive heart failure, carotid endarterectomy, or peripheral vascular procedures such as bypass or angioplasty. With 5,000 participants, the study has a 90% probability of detecting an 18% difference in major cardiovascular disease events between the two intervention groups.