Exquisite control of biological function is achieved via tight regulation of the catalytic and binding activities of cellular proteins. The mechanistic details of protein regulation vary from targeted chemical modification of amino acid side chains (1) to the quite drastic global unfolding of an entire polypeptide chain (2). Peptidyl prolyl cis/trans isomerization is emerging as a potentially general mechanism for the control of protein function (3). While most structures of native, folded proteins reveal peptidyl-prolyl imide bonds that adopt either the cis or trans conformation, there are a growing number of folded proteins that exhibit conformational heterogeneity about one or more peptidyl-prolyl bonds. Unlike covalent modification or global unfolding, proline isomerization is an intrinsic conformational exchange process that has the potential to direct ligand recognition and to control protein activity within the confines of the native state.