Abstract: A modular gas meter comprising a basic meter module, including a solenoid operated gas valve, a gas flow measurement arrangement and a gas valve control system, including a first optical interface which facilitates the transmission of control data to or from the control system, which basic module is adapted to be operatively combined with an add-on module so as to extend meter functionality, which add-on module comprises a module control system, which operates to control functionality and which includes a second optical interface, arranged for communication with the first optical interface when the two modules are mutually operatively combined, so that when functionality demands, an appropriate request signal is transmitted via the optical interfaces to request opening of the gas valve, and a manually operated switch to which the module control system is responsive for sending an enable signal via the interfaces to the valve control system, whereby operation of the gas valve to effect gas supply via the meter