The paper introduces a novel communication system called Ambient Backscatter, which enables two devices to communicate using ambient radio frequency (RF) signals as the sole source of power. This approach leverages existing TV and cellular transmissions to eliminate the need for wires and batteries, enabling ubiquitous communication in previously inaccessible locations. The key innovation is Ambient Backscatter, a communication primitive where devices communicate by backscattering ambient RF signals, which is orders of magnitude more power-efficient than traditional radio communication. The design avoids the expensive process of generating radio waves and does not require a dedicated power infrastructure. To demonstrate the feasibility of Ambient Backscatter, the authors prototype devices that can harvest energy from TV signals, transmit and receive data, and operate with touch sensors and LEDs. The prototype achieves information rates of 1 kbps over distances of 2.5 feet outdoors and 1.5 feet indoors. Two proof-of-concept applications are implemented: a bus pass that can transfer money to other cards and a grocery store application where item tags can detect if an item is placed in the wrong shelf. The paper also discusses the challenges of designing Ambient Backscatter systems, including the need to extract information from uncontrollable RF signals, the requirement for ultra-low-power devices, and the need for distributed multiple access protocols. The authors present a network stack that enables multiple Ambient Backscatter devices to coexist, including energy detection and carrier sense mechanisms. The prototype is evaluated in various environments, showing that it can operate battery-free at distances up to 6.5 miles from the TV tower and does not interfere with off-the-shelf TV receivers.The paper introduces a novel communication system called Ambient Backscatter, which enables two devices to communicate using ambient radio frequency (RF) signals as the sole source of power. This approach leverages existing TV and cellular transmissions to eliminate the need for wires and batteries, enabling ubiquitous communication in previously inaccessible locations. The key innovation is Ambient Backscatter, a communication primitive where devices communicate by backscattering ambient RF signals, which is orders of magnitude more power-efficient than traditional radio communication. The design avoids the expensive process of generating radio waves and does not require a dedicated power infrastructure. To demonstrate the feasibility of Ambient Backscatter, the authors prototype devices that can harvest energy from TV signals, transmit and receive data, and operate with touch sensors and LEDs. The prototype achieves information rates of 1 kbps over distances of 2.5 feet outdoors and 1.5 feet indoors. Two proof-of-concept applications are implemented: a bus pass that can transfer money to other cards and a grocery store application where item tags can detect if an item is placed in the wrong shelf. The paper also discusses the challenges of designing Ambient Backscatter systems, including the need to extract information from uncontrollable RF signals, the requirement for ultra-low-power devices, and the need for distributed multiple access protocols. The authors present a network stack that enables multiple Ambient Backscatter devices to coexist, including energy detection and carrier sense mechanisms. The prototype is evaluated in various environments, showing that it can operate battery-free at distances up to 6.5 miles from the TV tower and does not interfere with off-the-shelf TV receivers.