In this talk, I will present the fundamental understanding of giant magnetoimpedance (GMI) and the development of soft ferromagnetic amorphous and nanocrystalline materials (e.g. microwires, ribbons, and thin films) exhibiting GMI effects for use in high-performance magnetic sensor devices. In particular, I will discuss a new concept of incorporating soft ferromagnetic glass-coated microwires into fiber-reinforced composite as means of sensing stress and external magnetic fields through changes in impedance, which offers an alternative to optical fibers for self-monitoring composites. A new method of using the microwires as a microwave absorber for fabrication of a fiber Bragg grating-based microwave energy sensor with improved sensitivity and less perturbation of the microwave field will be presented. Finally, I will present a novel biosensing platform that integrates the GMI sensor with functionalized superparamagnetic nanoparticles for a highly sensitive, simple, and quick detection of cancer cells and biomolecules.