The first successful nuclear magnetic resonance (NMR) experiments were reported by two independent groups in the United States (Bloch et al., 1946; Purcell et al., 1946). In 1952, Purcell and Bloch received the Nobel Prize for their observations. Since chemical shift phenomena were discovered in the 1950s, NMR techniques were developed as important tools for chemical analyses. The early experiments were limited in scope by the relatively poor instrumentation available at that time. In the late 1960s, superconducting magnets were introduced to NMR experiments and revolutionized the scope of NMR together with the emergence of Fourier transform NMR (Ernst and Anderson, 1966). Lauterbur (1973) published the first NMR image of two small tubes of water. By 1980, the clinical evaluation of MRI had begun, and since that time, there have been continuing developments in instrumentation and applications that have led to where we are today. MRI is now established as an important modality in medical practice. The application techniques of MRI have diversified in recent years. Those techniques are, for example, magnetic resonance angiography (MRA), perfusion and diffusion imaging, and functional neuroimaging (fMRI). Scanning time has been dramatically shortened. Because there are clear attractions in using noninvasive methods for the study of living systems, those MRI techniques that investigate the brain and tissue characteristics appear to have a very bright future. Therefore, in this chapter, we give an overview of the principles of MRI techniques.