The chemistry of esters of hydrazonoic acids has gained increased interest in both synthetic organic chemistry and biological fields. Since a large number of developments in the use of such esters, a review of such developments, covering the literature up to mid 2005, seems to be of considerable value. The present review presents their structural features, nomenclature, synthetic methods and chemical reactions. The utility of the latter reactions for synthesis of various heterocyclic ring systems including mono-, bi-, tri-, tetra-, and penta-heterocycles. In addition, their silicon chelates and biological applications are presented.