Abstract: A gas flow detection device having an inlet for connection to a source of gas and an outlet for connection to a gas-using appliance is disclosed. Gas for feeding a pilot flame of the appliance flows through a permanently open orifice. When the appliance is switched on the pressure at the devices outlet drops and a piston which is in a tube is lifted by gas pressure at the device's inlet. Gas then flows from the inlet to the outlet. A light source and a light sensitive cell provide a signal that indicates that the piston has lifted. A monitoring system which makes use of the signal provided by the cell is also disclosed.