Hydrazine reagents are a well-known group of derivatizing agents for the determination of aldehydes and ketones in liquid and gaseous samples. Within this article, the most important hydrazine reagents are critically summarized, and their major applications in different fields, including environmental analysis, food chemistry and industrial analysis are introduced. As 2,4-dinitrophenylhydrazine (DNPH) is the basic reagent for several international standard procedures, its properties are discussed in detail. Particular focus is directed on the chemistry of the hydrazine reagents, and chemical interferences are considered. Recent methods for the determination of various oxidants using hydrazine reagents are presented as well. Due to limited space, this review does not cover the related field of carbohydrate analysis, although many chemical aspects are similar.