We consider the problem of providing ubiquitous yet affordable Internet connectivity to devices at home, at work, and on the move. In this context, we take advantage of two significant technology trends: the commoditization of WiFi WLAN technology and the rapid growth of cellular data services. We propose an architecture called Cool-Tether that harnesses the cellular radio links of one or more mobile smartphones in the vicinity, builds a WiFi hotspot on-thefly, and provides energy-efficient, affordable connectivity. Prior approaches to supporting such a tethered mode operation have relied on the WiFi ad hoc mode, which impedes the key goal of conserving battery energy on mobile phones. To address the challenges of energy efficiency, Cool-Tether carefully optimizes the energy drain of the WAN (GPRS/ EDGE/ 3G) and WiFi radios on smartphones. In particular, Cool-Tether employs a cloud-based gatherer and an energy-aware striper that exploit the unique energy characteristics of the WAN rad...