In mammals the M1 aminopeptidase family consists of nine different proteins, five of which are integral membrane proteins. The aminopeptidases are defined by two motifs in the catalytic domain; a zinc binding motif HEXXH-(X18)-E and an exopeptidase motif GXMEN. Aminopeptidases of this family are able to cleave a broad range of peptides down to only to a single peptide. This ability to either generate or degrade active peptide hormones is the focus of this review. In addition to their capacity to degrade a range of peptides a number of these aminopeptidases have novel functions that impact on cell signalling and will be discussed.