Since the discovery of the magnetoimpedance (MI) effect just over a decade ago, international research interest into the giant magnetoimpedance (GMI) effect has been growing. This article aims to provide a comprehensive summary of the GMI topic, encompassing fundamental understanding of the GMI phenomena, the processing and properties of GMI materials and the design and application of GMI-based magnetic sensors. The paper starts with the definition of GMI and an assessment of the current theoretical understanding of the frequency dependence of GMI. Then a detailed description of processing methods for the production of amorphous and nanocrystalline GMI materials in the form of wires, ribbons and thin films is given, with an examination of the advantages and disadvantages of each technique. Properties of existing GMI materials including magnetic, mechanical, electrical and chemical properties are described, and a correlation between domain structures and magnetic properties is established. The influences of measuring and processing parameters on the GMI effect are systematically analysed and the underlying physical origins of hysteretic and asymmetric phenomena of GMI are explained. This enables the selection of optimal conditions to design high-performance GMI-based sensors. After discussing the material selection criteria, a range of candidate materials are evaluated and nominated for the design of GMI-based sensors. Finally, a variety of potential applications of GMI-based magnetic sensors are presented with an outlook of future research development in this field.