This paper reviews the basics of the interferometric fiber-optic gyroscope: fundamental principle based on Sagnac effect, importance of reciprocity and single-mode propagation, analysis of coherence and polarization problems, signal processing techniques. It also describes the technological progresses of guided-wave components (single-mode optical fiber, semi-conductor diode source, integrated optics, in-line fiber components). Finally, recent trends like multi-axis configurations and rare-earth doped fiber source are presented.