This article gives a systematic review of the many NMR experiments in solid helium to date, emphasizing the view-point that the results may be interpreted primarily in terms of the effects of three fundamental excitations in the solid: the vacancy waves; the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ tunneling interaction or exchange; and the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ tunneling interaction or mass fluctuation waves.

DOI:https://doi.org/10.1103/RevModPhys.43.532