"Nuclear magnetic resonance (nmr) is playing an increasingly important role in the characterization of the organic constituents of fossil fuels. The nmr techniques that are currently being applied to fossil fuel characterizations utilize both the 1H and 13C nuclides and can conveniently be divided into those that apply to liquid samples and those that apply to solids. For solids one has to contend with line-broadening influences of dipole-dipole interactions and chemical shift anisotropies (especially for carbon) and with long spin-lattice relaxation times; some recent advances have largely circumvented these problems for 13C.14-20 For liquids, rapid isotropic molecular tumbling averages dipolar splitting to zero and the chemical shift tensor to its isotropic average, and gives rise to relatively efficient spin-lattice relaxation (permitting one to reduce the wait time between pulse repetitions)."