This category covers a wide potential range of sensors that may be used to determine the structural behavior of buried structures. In person-entry structures, these can be attached directly to the inner surface of the structure and monitored by direct connection. Examples include crack width gauges, strain gauges, acoustic and vibration sensors and fiber optic sensor systems. Many are useful for localized application but are not suitable for broad deployment as a systematic structural response system – especially in non-person-access piping. An exception is the acoustic monitoring for prestressing wire breaks in prestressed concrete pipe since this can be done from only limited points of access. Nevertheless, structural response sensor systems represent a promising area for future development and fiber optic sensor systems, for example, have the capability to monitor pipe behavior continuously along the length of a pipe from a single access point.