Interferometric fiber sensors using highly magnetostrictive amorphous metals have demonstrated the capability to detect magnetic fields with high resolution. These sensors have potential applications in the areas of geophysics, medical technology, and a variety of commercial and military fields. This paper discusses the magnetoelastic properties of amorphous metal transducers in terms of the coherent rotation model for a magnetic domain. The general operating principles of fiber optic magnetic sensors, including single and multiple axis devices, are developed and theoretical limitations arising from phase noise, shot noise, and thermal noise are discussed. Finally, the results of a number of experiments with fiber magnetometers for both low-(⩽ 100 Hz) and high-frequency (> 100 Hz) magnetic field measurements are presented. These include high-resolution measurements on laboratory devices, testing of a compact three-axis magnetic heading sensor, and a demonstration of fiber optic detection of the mixing of spin wave modes in a metallic glass sample.