Peptides have enormous therapeutic potential but traditionally suffer from poor bioavailability. Macrocyclization is a key strategy for enhancing the drug-like properties of peptides, and new cyclization methods1 are in exceedingly high demand in the pharmaceutical industry. Herein, a versatile strategy for macrocyclization inspired by the biosynthesis of non-ribosomal imine natural products is presented. Such macrocyclic natural products possess an intriguing head-to-tail imino linkage which forms upon spontaneous cyclization of the linear peptide amino aldehyde.2 In this study,3 imine macrocyclization is explored as a general, reversible mode of peptide cyclization, and the reactivity of the imine linkage toward inter- and intramolecular nucleophiles is exploited for late-stage peptide diversification. The cyclization proceeds in aqueous solution and in the absence of protecting groups. Application to the synthesis of four peptide natural products and the structural interrogation of relevant linear and macrocyclic peptides will also be discussed.