Peptide sequences capable of binding to insulin and/or insulin-like growth factor receptors with either agonist or antagonist activity and identified from various peptide libraries are disclosed. This invention also identifies at least two different binding sites, which are present on insulin and insulin-like growth factor receptors, and which selectively bind the peptides of this invention. As agonists, the peptides of this invention may be useful for development as therapeutics to supplement or replace endogenous peptide hormones. The antagonist peptides may also be developed as therapeutics.