The Bodybugg uses 4 main features to keep track of your activities (it's much more than just a pedometer). 1. Accelerometer - a two-axis micro-electro-mechanical sensor that measures motion 2. Heat flux - a sensor measures how much heat the person's body is giving off 3. Galvanic Skin Response - measures electrical current across the skin, essentially this aspect is measuring how much you sweat 4. Skin temperature - tracking a person's core body temperature gives the Bodybugg an indication of how much physical exertion the person is doing