An anode monitoring system for monitoring the integrity of anodes (2) provided on a metallic structure such as a pipeline system (1). A signal circuit (Sn) including the pipeline (1) and one of the anodes (2) is set up and appropriate signals are looked for at a central station (4). Reception of the signal at the central station (4) is dependent on the integrity of the anode (2) under inspection at that time. If the anode (2) is missing or defective the expected signal is not received and thus the fault in the anode (2) can be detected. A notch filter (5) is inserted in series between each anode (2) and the pipeline (1). The filter (5) provides a high impedance which can be signalled across but does not interfere with the cathodic protection system.