This chapter provides an overview of the fundamental elements of magnetic resonance imaging (MRI). Four terms describe the magnetic properties of materials, such as contrast agents, used in MRI. These terms are diamagnetism, paramagnetism, superparamagnetism, and ferromagnetism. The persistence of magnetization when the external magnetic field is removed distinguishes ferromagnetic materials from paramagnetic materials. To be useful for MRI, the proton must have spin angular momentum, in addition to the nuclear magnetism. Echo time (TE) and repetition time (TR) are basic parameters of image acquisition. Improvement in the magnitude of the MR signal can improve signal-to-noise ratio (SNR). Magnetic resonance angiography (MRA) uses the same MRI system and methods to make images of blood vessels. The most common MRA technique is based on the time-of-flight (TOF) effect, where blood protons flowing into the slice during the acquisition yield very high signal, but signal from stationary protons is suppressed.