The NMR signals of isotopically or chemically dilute nuclear spins S in solids can be enhanced by repeatedly transferring polarization from a more abundant species I of high abundance (usually protons) to which they are coupled. The gain. in power sensitivity as compared with conventional observation of the rare spins approaches N/ (I + l)'Y/IN s S (S + I}'Y/, or '" 10 3 for S = 13C, 1= IH in organic solids. The transfer of polarization is accomplished by any of a number of double resonance methods. High-frequency resolution of the S-spin signal is obtained by decoupling of the abundant spins. The experimental requirements of the technique are discussed and a brief comparison of its sensitivity with other procedures is made. Representative applications and experimental results are mentioned.

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