Acetylation (or in IUPAC nomenclature ethanoylation) describes a reaction that introduces an acetyl functional group into a chemical compound. Deacetylation is the removal of an acetyl group.

Acetylation refers to the process of introducing an acetyl group (resulting in an acetoxy group) into a compound, namely the substitution of an acetyl group for an active hydrogen atom. A reaction involving the replacement of the hydrogen atom of a hydroxyl group with an acetyl group (CH₃ CO) yields a specific ester, the acetate. Acetic anhydride is commonly used as an acetylating agent reacting with free hydroxyl groups. For example, it is used in the synthesis of aspirin and heroin.