The theory of solid-state nuclear magnetic resonance (NMR) spectroscopy is reviewed, with specific discussions of magnetic interactions in the solid state. Each magnetic interaction (Zeeman, dipole-dipole, chemical-shift, spin-spin, and quadrupolar) is addressed and manifestations of these interactions in the solid state NMR spectrum are explained. The techniques of high-power decoupling, magic-angle spinning, and cross-polarization, used to acquire highly resolved solid-state NMR spectra, are also illustrated. Application of solid-state NMR to pharmaceutical problem solving and methods development is then briefly reviewed.