Advances in energy harvesting and wireless energy transfer are redefining the scope and extent of the energy constraints in low-power embedded computing. It now becomes conceivable to power not just RFID-scale devices out of harvested or wirelessly-transferred energy, but also more powerful devices operating in applications such as smart buildings, factory automation, and mobile health. However, energy provisioning from ambient energy harvesting or wireless energy transfer is generally erratic. Thus, devices need to cope with highly variable, yet unpredictable energy supplies across both space and time. The challenges arising in this field are generating an entirely new stream of research from the hardware up to the application layers, and novel solutions are being devised to attack these unique problems.