An apparatus (1) for determining a position of a disturbance to an optical fibre assembly (2) comprises two monitoring units (6, 8), each comprising a respective light detector (12a, 12b) and a respective clock (24a, 24b). The first monitoring unit (6) receives a first digital optical signal from the optical fibre assembly (2). The second monitoring unit (8) receives a second digital optical signal from the optical fibre assembly (2). The apparatus (1) monitors the state of polarisation of the first and second digital optical signals and determines respective times at which a change occurs in the state of polarisation of each signal, the change arising from a disturbance to the optical fibre assembly (2). The apparatus (1) uses said times to determine a position of the disturbance.