Detecting human movement using WiFi commercial off-the-shelf devices can be effected by exploiting Channel State Information (CSI). CSI is part of the WiFi protocol and models the propagation of a WiFi signal from the transmitter to the receiver, supporting many subcarriers due to the Orthogonal Frequency Division Multiplexing (OFDM) principle. Taking profit of the information provided by the CSI it is possible to detect an intruder in a room or even detect the intruder through the wall. The Human Presence Detector (HPD) uses this technology to detect the presence of a human in movement.