The immunophilins are proteins which are capable of influencing the immune response in combination with an immunosuppressive drug. Their natural function, however, is mainly the cis/trans isomerization of peptidyl-prolyl bonds in other proteins. This review lists all immunophilin structure coordinates currently available in the RCSB protein data bank and highlights the key active-site factors that define their catalytic and immunological action. In addition, an overview of biologically-relevant functions is provided for various immunophilin members.