There has been intense research in the area of fiber-optic sensors over the last decade. A wide range of application areas have been identified and the technology has now matured to the point where the sensor systems are commercially available from a number of companies. Furthermore, many other companies and research establishments are actively involved in the field-testing of prototype sensors systems. This paper discusses the basic principles and applications of optical-fiber sensor technology, outlining the various sensing mechanisms which can be utilized to influence the propagation of light in an optical fiber and the various basic forms of sensor types—intensity, phase-modulated (inferometric), intrinsic, and extrinsic. Sensors for the measurement of temperature, pressure, liquid level, chemical parameters, acoustic, magnetic field, and rotation (gyroscope) are briefly described.