Over the past decade, personal computers have been transformed into small, often mobile devices that are rapidly multiplying. Aside from the ever-present smartphone, a growing set of computing devices has become part of our everyday world, from thermostats and wristwatches, to picture frames, personal activity monitors, and even implantable devices such as pacemakers. All of these devices bring us closer to an “Internet of Things,” but supplying power to sustain this future is a growing burden. Technological advances have so far largely failed to improve power delivery to these machines. Power cords tie devices down, prohibiting their free movement, while batteries add weight, bulk, cost, the need for maintenance, and an undesirable environmental footprint. Fortunately, running small computing devices using only incident RF signals as the power source is increasingly possible. We call such devices RF-powered computers. As might be expected, the amount of power that can be harvested from typical RF signals is small. However, the energy efficiency of the computers themselves has improved exponentially for decades-a lesser-known consequence of Moore's law. This relentless improvement has recently brought the power requirements of small computational workloads into the microwatt realm, roughly equal to the power available from RF sources in practical settings.