The present invention relates to testing a passive optical network having a head end and a plurality of terminals, which terminals are connected to the network at a respective plug and socket arrangement located at customer premises. A test device is provided having a standard plug for connection in place of a terminal in the socket of the relevant customer premises. An identification device is provided in the socket. The test device is arranged to (i) read the identity of the identification device and (ii) test the line and transmit a result to the head end together with the identity so that the line to the customer premises can be tested remotely. When the line has been tested, the test device can be removed. Because the test device uses an existing connection, the optical (insertion) loss is reduced.