There is both commercial and practical demand to extend the applications for which fibre optic sensors are used. This thesis describes the development of fibre optic sensors and systems for security and defence applications, based on established fibre-optic hydrophone I technology. Dynamic acoustic and seismic signals have been detected with sensor arrays, and used to demonstrate the detection of targets. Achievements have been made in the development and characterisation of entirely novel designs of fibre optic accelerometers and microphones, with the demonstration of arrays of these in practical application to intruder detection and the location of distant acoustic sources. Several prototypes of both acoustic and seismic sensors have been developed. Research efforts focused on designing ,,/ . /~nd establishing the perfonnance of these sensors, then refining the strongest designs for incorporation into arrays of sensors. These arrays have been deployed in environments representative of real operational deployment situations, extending the proof of principle established in the laboratory to demonstration in the field. Specifically, six entirely novel· fibre optic microphone designs are presented, concerning both intensity and interferometric schemes, and evaluated for responsivity and suitability to the location of acoustic targets in a battlefield application. The strongest of these is identified to be a foam-backed fibre wrapped mandrel, of simple design and high responsivity, and readily multiplexed into and interrogated with an inter~rometric system. An array of such foam-backed mandrel microphones is also demonstrated in the field and shown to have very good target location capability at ranges in excess of lkm. In other work, an entirely novel design of fibre optic accelerometer is presented, and theresponse of this is characterised in detail, in terms of both responsivity and the impact of the sensor set-up condition on responsivity. A buried array of such accelerometers combined with distributed cable sensors is demonstrated in a field deployment detecting the presence of signals from an intruder in the region of the array.