This chapter describes magnetic resonance imaging (MRI) principles and the main components of an MRI scanner, in particular those responsible for the generation of magnetic fields and signal acquisition. The static magnetic field is employed both for polarizing the sample and imposing the frequency of excitation and signal acquisition. These two aspects determine the specifications and constraints of the magnet. The chapter explains how the magnetization relaxation to equilibrium can be modulated to obtain information on spatial localization or tissue composition. It discusses the principles of encoding an nuclear magnetic resonance (NMR) image performed using gradients of magnetic fields in a rather succinct way. In order to perform the spatial encoding of information, additional magnetic fields have to be superposed to the main static magnetic field. The NMR signal measured by a magnetic flux detector is obtained after the steps of amplification, demodulation, sampling and digitization via analog or digital converters.