Excessive body weight adds to the compressive forces on the knee joint and is a risk factor for OA. While there is no cure for OA, weight loss is the best nonpharmacologic way to slow OA progression. The amount of weight loss necessary to alter OA progression is unknown. The purpose of this study is to compare the effects of significant dietary restriction, exercise, and a combination of both in reducing knee inflammation and compressive forces in obese and overweight adults with knee OA. The study will also examine how diet and exercise affect everyday function and pain levels, as well as the extent of weight loss necessary for slowing OA disease progression.