A new displacement monitoring system based on low coherence interferometry using standard telecommunication fibers is presented. The measuring system is especially designed for the needs and conditions encountered in the field of civil engineering. The system features a precision of 10 micrometers to 100 micrometers (depending on the sensor type) over a measuring length up to 100 m, an operational range of 70 mm, stability over long periods (at least two years) and insensitivity to aging of the fiber and connector losses. The results of laboratory and field tests show that fiber optic extensometers are promising tools for the long term monitoring and survey of civil engineering structures. The theory and the optical set-up are briefly described as well as different sensor types that have been developed for various applications (bridges, tunnels, retaining walls, piles, anchorage cables) and tested in the field and in the laboratory. To validate the technique and calibrate the sensors laboratory testing has been done for short sensors (0.5 - 2 m) at IMM and for long sensors (2 - 10 m) at LMS. The results of these tests are presented and discussed in detail. IMM made several tests with fibers embedded in concrete beams and slabs measuring deformations due to shrinkage, creeping, and imposing traction, pressure and bending to the structure. Field tests on a rehabilitated bridge are also presented. Using a 10 m long calibration bench, LMS tested several fiber set-ups using different fixation techniques and imposed displacements on groups of extensometers with different lengths.