"The theory of hindered motions of small molecules and molecular fragments in crystals and liquids allows one to obtain detailed knowledge of both the crystal structure and the dynamical behavior of molecules. Application of the theory to describe such phenomena as NMR relaxation rates and spectral line broadening expands the power of many spectroscopy techniques. The author compares this theoretical approach with recent experimental results. Specific topics covered include the extended angular jump model, angular autocorrelation functions, dielectric and optical spectroscopy applications, and nuclear magnetic resonance spin-lattice relaxation applications"--