On a long, repeatered, submarine-cable link, it is essential that facilities are provided for the terminal-station staff to be able to monitor the performance of each repeater in the link. The paper describes the two types of monitoring equipment that have been designed and installed at the British and Canadian terminal stations of the CANTAT 90-repeater submarine link. One type of equipment uses a radar-type technique, and relies on the natural non-linearity of the repeaters for the return signals; it provides, in effect, a means for monitoring the non-linearity of the repeaters. The other equipment uses a selective frequency-changing device in each repeater for measuring from one terminal station the loop gain to, and the noise—or intermodulation-generated by, any selected repeater.