Geminal diols or gem-diols are the product of the addition of water to a carbonyl group of either an aldehyde or a ketone. These compounds are rarely stable and seldom found in the liquid or solid state since the moisture balance is largely dependent on the structure. However, the family of organic chemical compounds with this gem-diol moiety has increased over the last years due to the multidisciplinary research studies dedicated to the chemical characterization of this functional group, including the assessement of those factors that confer stability to these molecules. Gemdiols participate in beneficial chemical reactions involved in the development of new therapeutic strategies. Gem-diols may also be of environmental concern; particularly, imidazole and pyridine molecules containing either carbonyl or its hydrated groups are widely used for the synthesis of metal complexes with relevance in Medicinal and Coordination Chemistry due to the ability of gem-diols to coordinate different transition metals ions (copper and cobalt) and heavy metals ions (lead, chrome and arsenic), thus being attractive molecules to be employed in the synthesis of new polymeric materials and/or metal complexes for the catalytic removal of pollutants and the design of single-molecule magnets. This chapter will describe the spectroscopic techniques available for the identification of this functional group in heterocyclic compounds containing carbonyl groups, such as pyridinecarboxaldehyde, vitamin-B6- related compounds and imidazolcaboxaldehydes, among others. The aim of this chapter is to show that the combination of both single-crystal X-ray diffraction and nuclear magnetic resonance allows determining the presence of the gem-diol group together with the factors governing the stability and the occurrence of this rare functionalization. This chapter will also focus on and discuss the applications of this chemical group in organic synthesis, atmospheric processes, and their effect on odorant receptors and on the catalytic sites of enzymes. © 2020 by Nova Science Publishers, Inc. All rights reserved.