Imagine using a health bracelet that tracks your blood pressure and glucose level that you do not have to charge for the next 20 years. Imagine sensors attached to honeybees helping us understand how they interact with their environment or bio-absorbable pacemakers controlling heart rate for 6–8 months after surgery. Whether submillimeter-scale “smart dust,”25 forgettable wearables, or tiny chip-scale satellites, the devices at the heart of the future of the Internet of Things (IoT) will be invisible, intelligent, long-lived, and maintenance-free. Despite significant progress over the last two decades, one obstacle stands in the way of realizing next-generation IoT devices: the battery. Batteries provide the convenience of a reliable energy supply but create a host of problems. Batteries limit device lifetime and yield high maintenance costs. As device size has continues to scale down, battery density scaling has not kept pace. As IoT device applications demand more computational capabilities, energy limits lifetime to weeks or months. Even rechargeable batteries have a limited lifetime, wearing out after 300–500 charge cycles. As we move toward a future with trillions of IoT devices,a replacing trillions of dead batteries and devices will be both prohibitively expensive and irresponsible. A future IoT with trillions of new battery-powered devices would create an environmental catastrophe.