Weight control is a major health problem in the United States. This project applies basic behavioral processes to support achieving and maintaining exercise levels which will affect physical health and maintenance of emotional well-being. A user-friendly monitor of physical activity will provide feedback to the individual relevant to natural activity levels. The monitor is designed to assist in supporting improved mental health through less depression and better weight control and to maximize healthful behaviors. Studies indicate that behavior change with sustained increases in physical activity are important to maintain weight loss; moreover, increasing exercise through natural, 'lifestyle' activities appears preferred for lasting changes. Similarly, higher levels of naturally occurring locomotive activity appear to decrease depression and improve health. The studies and clinical work on altering levels of naturally occurring activity suffer from lack of combining reliable, direct measurement of activity with information feedback needed to shape behavior. In Phase I, two types of prototype feedback monitors were developed and field tested demonstrating feasibility of use to increase natural exercise. Phase II will: (1) develop both a professional and a consumer feedback monitor; (2) establish relationships between activity measurements and behavior; and (3) explore methods for and benefits from using these monitors with weight loss treatments.