Because they form a strong electrophile when treated with Lewis acids, acyl halides are commonly used as acylating agents. For example, Friedel–Crafts acylation uses acetyl chloride () as the agent and aluminum chloride () as a catalyst to add an acetyl group to benzene:
This reaction is an example of electrophilic aromatic substitution.
Acyl halides and acid anhydrides of carboxylic acids are also common acylating agents.  In some cases, active esters exhibit comparable reactivity. All react with amines to form amides and with alcohols to form esters by nucleophilic acyl substitution.
Acylation can be used to prevent rearrangement reactions that would normally occur in alkylation. To do this an acylation reaction is performed, then the carbonyl is removed by Clemmensen reduction or a similar process.