AbstractThe functionalization of peptides and proteins by aldehyde groups has become the subject of intensive research since the discovery of the inhibition properties of peptide aldehydes towards various enzymes. Furthermore, peptide aldehydes are of great interest for peptide backbone modification or ligation reactions. This review focuses upon their synthesis, which has been developed following two main strategies. The first strategy consists of prior synthesis of the peptide, followed by the introduction of the aldehyde function. The second possible strategy uses α‐amino aldehydes as starting materials. After protection of the aldehyde, peptide elongation occurs. At the end of the synthesis, the aldehyde function can be unmasked. Copyright © 2006 European Peptide Society and John Wiley&Sons,