Location information is a key contextual element for 911 emergency response and other emerging applications. We have develop the first accurate localization systems based on GSM and CDMA cell phone signals, which can be used in indoor environments where GPS does not work well. Before this work, it was widely presumed that the large cell size of cell phone systems made them inherently inaccurate for indoor localization. Instead, our research showed that cell phone based localization can achieve median accuracies of 5 meters in large multi-floor structures. The key to high accuracy is the use of wide signal-strength fingerprints, which include signals that are strong enough to be detected, but are too weak to be used for communication; the higher dimensionality introduced by the additional signals dramatically increases accuracy. Cell phone based localization has the advantage that it uses the phone’s existing hardware removing the need for additional interfaces.