Hydrazones are a class of organic compounds with the structure R¹R²C=N−NH₂. They are related to ketones and aldehydes by the replacement of the oxygen =O with the =N−NH₂ functional group. They are formed usually by the action of hydrazine on ketones or aldehydes.

Hydrazine, organohydrazines, and 1,1-diorganohydrazines react with aldehydes and ketones to give hydrazones.

Phenylhydrazine reacts with reducing sugars to form hydrazones known as osazones, which was developed by German chemist Emil Fischer as a test to differentiate monosaccharides. Hydrazones having 1,3-diketomoiety are also known in literature.

More generally, diazonium ions react with carbon acids in the Japp-Klingemann reaction to give hydrazones via tautomeric rearrangement of a diazo intermediate.

Hydrazones are the basis for various analyses of ketones and aldehydes. For example, dinitrophenylhydrazine coated onto a silica sorbent is the basis of an adsorption cartridge. The hydrazones are then eluted and analyzed by high-performance liquid chromatography (HPLC) using a UV detector.^{[citation needed]}

The compound carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (abbreviated as FCCP) is used to uncouple ATP synthesis and reduction of oxygen in oxidative phosphorylation in molecular biology.

Hydrazones are the basis of bioconjugation strategies. Hydrazone-based coupling methods are used in medical biotechnology to couple drugs to targeted antibodies (see ADC), e.g. antibodies against a certain type of cancer cell. The hydrazone-based bond is stable at neutral pH (in the blood), but is rapidly destroyed in the acidic environment of lysosomes of the cell. The drug is thereby released in the cell, where it exerts its function.

Hydrazones are susceptible to hydrolysis: