This Account examines the role of electron-nuclear double resonance (endor) spectroscopy in furthering our understanding of how metal ions function in biological systems. It briefly describes endor and electron spin-echo envelope modulation (eseem) spectroscopies and then illustrates the uses of endor with several case studies from our own research: cytochrome c peroxidase compound ES; ribonucleotide reductase intermediate X; allylbenzene-inactivated chloroperoxidase; the role of the [4Fe-4S](+) cluster in enzymes of the "radical S-adenosylmethionine" superfamily; dioxygen activation by heme enzymes. Finally, it briefly considers future developments.