An aldehyde, in organic chemistry, is defined as an organic compound that possesses a functional group characterized by the structure R−CH=O.

Aldehydes display diverse properties that are determined by the specific structure of the molecule. Smaller aldehydes tend to be more soluble in water, with formaldehyde and acetaldehyde being fully soluble. Aldehydes that are volatile in nature typically possess pungent odors.

Aldehydes are characterized by a carbon center that is bonded by a double bond to oxygen and by single bonds to hydrogen and another substituent, which can be a hydrogen atom in the case of formaldehyde.

Aldehyde dehydrogenase enzymes in the body can metabolize certain aldehydes. However, some aldehydes are linked to toxicities that can lead to neurodegenerative diseases, heart disease, and certain types of cancer.

Due to the high reactivity of the formyl group, aldehydes are not prevalent in several natural building blocks such as amino acids, nucleic acids, and lipids. However, many sugars are derived from aldehydes, with these aldoses existing as hemiacetals that act as a disguised form of the parent aldehyde.