This document discusses Wi-Vi (Wireless Vision) technology, which uses Wi-Fi signals to detect motion behind walls. Wi-Vi was developed by researchers at MIT in 2013. It consists of a transmitter that sends Wi-Fi signals and a receiver that detects the reflected signals. When people move in the detection area, their motion causes changes in the reflected signals, allowing the system to identify the number of moving objects, their locations, and even simple gestures. Wi-Vi has potential applications in security, emergency response, and interfaces that can detect gestures from the other side of a wall. The document outlines the basic principles and components of Wi-Vi systems and provides examples of its capabilities and possible future developments.