The optical fibre Sagnac interferometer was first demonstrated over 25 years ago. Immediately its potential for gyroscopic measurements became apparent and since the first demonstration substantial research and development investment has evolved a diversity of rotation measuring instruments. The fibre Sagnac interferometer has, however, also ventured into unexpected domains. The fibre loop mirror has become the ubiquitous reflector. Sagnac-based intruder alarms, hydrophones, geophones and current measuring systems have emerged. The Sagnac interferometer has expanded from the rotation measuring instrument into a very versatile sensing tool. Indeed, it is arguably the most successful of optical fibre sensing technologies. In this paper, we review both the principles and applications of the fibre Sagnac interferometer. The background theory highlights the need to understand the conditions for reciprocity within the interferometer network. The applications range from the expected gyroscopes into novel hydrophone arrays and intruder detection systems. The basic interferometer is now well understood and the engineering required to realize useful and effective instruments has been carefully defined. Its versatility though continues to amaze even the most experienced practitioner and doubtless its applications potential will continue to expand.