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GPS is the most popular wireless‐signal‐based positioning system in use today, and is extremely useful in outdoor environments [5]. GPS satellites broadcast microwave signals to enable GPS receivers on or near Earth's surface to determine location, velocity, and time. The GPS system itself is operated by the US Department of Defense (DoD) for use by both the military and the general public. Unfortunately, GPS signals cannot penetrate into indoor environments due to obstacles that spread and attenuate the electromagnetic radio signals [6]. Thus, GPS cannot be used for localization in indoor environments. Fortunately, there are many other signals available in indoor locales that can be leveraged by solutions intended for indoor localization. This section reviews some of the more relevant signals that can be used for indoor localization. Figure 37.1 shows a taxonomy of the signals that are covered in more detail in the rest of this section.