This chapter presents an introduction to nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) in the context of the applications of these techniques to cartilage. The introduction (Section 3.1) contains a brief overview of the relationship between the two techniques, and the following sections discuss their physical basis. Section 3.2 presents the vector model of NMR, which provides a conceptually simple picture of magnetic resonance intended to be accessible to all readers. Section 3.3 introduces the quantum-mechanical apparatus of magnetic resonance. The emphasis is placed on the density-matrix description of NMR; this section should be accessible to researchers and students with a working knowledge of linear algebra and basic quantum mechanics. The physical basis of MRI is discussed in Section 3.4.