Chapter 12 studies the property of magnetic response functions of electrons probed by nuclear magnetic resonance (NMR) experiments. The Knight shift is shown to be proportional to the real part of the local magnetic susceptibility. The spin-lattice relaxation, on the other hand, provides an effective measure of the imaginary part of the susceptibility averaged by the interaction form factor over the whole Brillouin zone. The effect of impurity scattering, particularly the impurity induced resonance states, on the NMR spectra is discussed and compared with experimental results.