Hydroxynitrile lyases are valuable enzymes for asymmetric synthesis of cyanohydrins. These hydroxyl and nitrile–containing compounds are being used in production of very useful pharmaceuticals, agrochemicals, and other biologically active compounds using chemical or chemoenzymatic follow-up reactions in industry. Although a huge amount of information exists on the reaction parameters of these enzymes, including stability to pH and organic solvents, yield, reaction time, and valuable data on the enantiopurity of their products, cyanohydrins, there is a lack of update on the biochemistry, discovery, and engineering of the HNLs. Therefore, in the Introduction, we will have a look into these enzymes, cyanohydrins, and aldoxime-nitrile pathways. A brief view of functional groups and several examples of cyanohydrin-based chemicals and pharmaceuticals will also be described. Then we will present characteristics of many S- and R-selective HNLs with comparative tables for several enzymatic properties under biochemistry section. The methods of screening and discovery of these enzymes both from nature and a library of mutants will be described as well as their potential in the synthesis of chemicals. Cloning and expression of new HNLs will also be described under the discovery section. A pool of successful applications of protein engineering methods and the subsequent improvement in the properties of mutant HNLs will be reviewed in detail afterward.