This chapter presents a comprehensive review of magnetic sensors as a valuable reference for engineers involved in magnetic applications. These sensors typically share common laws, techniques, and components. Considering their long historical backgrounds, researchers have named and categorized various types of magnetometers and its gradiometers. From the author’s viewpoints, this chapter offers eight categories based on its fundamental. Broadly speaking, all the magnetic sensors are transducers, which convert the magnetic signal into the voltage signal. The common key techniques in magnetic sensors include flux concentration, modulation, and null detection. These are mentioned at corresponding parts. For a straightforward understanding, this chapter avoids using equations of the theoretical details and tries to provide simple schematics and resent reference books.