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This article has described the impetus for the world's largest application of wireless geolocation, specifically, the FCC's E-911 requirement in the United States. It has also discussed two basic approaches to wireless geolocation, time and power derived, and showed that U.S. wireless carriers have primarily chosen time-derived approaches to meet the E-911 requirements for their more than 200 million subscribers, primarily because of their superior accuracy. It has been shown that within the domain of time-derived approaches, there are two primary techniques that are used in the United States, network-based UTDOA and handset-based GPS/A-GPS, with each technique serving about an equal percentage of the population. The backup techniques for each primary technique were also addressed, and interestingly, they are also time derived. Additionally, this article has discussed the other important metrics for wireless geolocation, yield, and latency, and suggested that network-based U-TDOA is superior to other techniques because it meets many, or all, of the criteria for an optimum mission-critical safety and security application, such as the E-911 location system. It has reviewed the parameters that impact the accuracy for the technique of U-TDOA and presented measured results for network-based U-TDOA on a commercial GSM network. Finally, this article discussed other applications of network-based U-TDOA technology that add to the safety and security of the population that is covered.