The 5th generation mobile communica-tions aims at connecting everything and future Inter-net of Things (IoT) will get everything smartly con-nected. To realize it, there exist many challenges. One key challenge is the battery problem for small devices, such as sensors or tags. Batteryless backscatter, also referred to as or battery-free backscatter, is a new po-tential technology to address this problem. One early and typical type of batteryless backscatter is ambient backscatter. Generally, batteryless backscatter utilizes environmental wireless signals to enable battery-free devices to communicate with each other. These de-vices first harvest energy from ambient wireless sig-nals and then backscatter these signals so as to transmit their own information. This paper reviews the current studies about batteryless backscatter, including vari-ous backscatter schemes and theoretical works, and then introduces open problems for future research.