Since dehmelt observed the first pure quadrupole resonance signal in 1949, this technique has been developed to a high degree of sophistication for the investigation of bonding and structural features in compounds. Most of the early data were obtained on organic halides. More recently, nqr has been used to research inorganic bonding characteristics such as pi and o bonding in nitrogen compounds and transition metal complexes. The technique has also been extensively developed for mineralogical research, including recent applications for quantitative analysis of specific minerals in natural ore deposits. The major developments in instrumentation that were made in an effort to solve these problems are discussed.