Magnetic Resonance is increasing its importance as a diagnostic mean. For an effective and accurate MRI practice, new fundamental principles, which are at the basis of this phenomenon, must be acquired. In this chapter an extremely simplified overview of the physical basics of MRI is introduced, to help the readers understand the principles underlying this complex technique. Basic physics, T1 and T2 relaxation times, spin echo sequences, which can be acquired with the now available units, and tissue characteristics are illustrated. For those who are interested in a further and more detailed reading in this field, a bibliography is also quoted.