Nike+ Stand Alone Running Sensor Kit (2) Fits under sockliner of Nike+ ready shoes and syncs with your Nike+ SportWatch GPS, iPhone 3GS/4, or other Nike+ tracking device Sensor measures pace, distance, time elapsed, and calories burned, transmits wirelessly to your device for real-time feedback Works with Nike+ SportWatch GPS powered by TomTom; Nike+ SportBand; iPod nano, iPod touch 2G, iPhone 3GS, or iPhone 4 Requires Nike+ ready shoes; Mac OS X 10.3.9 or later, or Windows Vista, XP (SP2), Home, or Professional; Internet access to connect to Nikeplus.com