The present invention relates to conditions monitoring of structures forming part of a transport network, e.g. structural health monitoring of structures, especially tunnels by performing distributed acoustic sensing (DAS) on at least one optical fiber (104) deployed so as to monitor the structure (206) and detecting and analyzing the acoustic response to movement of traffic (205) on the network in the vicinity of the structure to detect the acoustic response (303, 304) of the structure. The acoustic response of the structure is then analyzed to detect any change in condition.