Glucagon and the glucagon-like peptides are encoded within a larger precursor, proglucagon. The proglucagon gene is expressed in the pancreas, intestine, and brain, giving rise to a single proglucagon mRNA transcript that is identical in all tissues. Tissue-specific posttranslational processing of proglucagon accounts for the different molecular forms of the glucagon-related peptides present in each tissue. Glucagon-like peptide I has been demonstrated to have potent insulinotropic properties, but the biological importance of other glucagon-related peptides remains unclear. The recent advances that have increased our understanding of the biology of glucagon and the glucagon-like peptides are reviewed.