An apparatus and a method for monitoring an optical transmission line. An optical pilot signal of a predetermined duration is transmitted along said optical transmission line and a return signal is sent back from a signal returner along the same transmission line corresponding to at least part of said optical pilot signal. The signal returner is positioned at a predetermined point along the optical transmission line. An optical detection apparatus detects said optical pilot signal and also detects said return signal. A monitoring unit receives detection signals from said optical detection apparatus and determins the time relationship between the predetermined duration of the optical pilot signal and the round-trip transit time of the optical pilot signal. A first monitoring signal is generated when the determined time relationship has a predetermined value, and at least one further monitoring signal is generated in other cases.