Magnetic resonance imaging (MRI) and spectroscopy (MRS) are noninvasive techniques that allow the characterization of morphology, physiology and metabolism in vivo. MRI and MRS have become techniques of choice in many pre-clinical and clinical applications. In this chapter, the basic principles and the instrumentation of MRI and MRS are described. Furthermore, the factors that influence the sensitivity are discussed and examples for the limit of contrast agent detection are given.