Cu(A) is a binuclear copper center that functions as an electron transfer agent, cycling between a reduced Cu(I)Cu(I) state and an oxidized mixed-valence Cu(+1.5)...Cu(+1.5) state. The copper ions are bridged by two cysteine thiolate ligands and form a copper-copper bond, the first reported of its kind in Nature. Such a "diamond-core" Cu(2)S(Cys)(2) structure allows an unpaired electron to be completely delocalized over the two copper ions and contributes to its highly efficient electron transfer properties. This review provides accounts of how the Cu(A) center was structurally characterized and highlights its salient spectroscopic properties. In the process, it introduces the Cu(A) center in four different systems-native protein systems, soluble protein truncates of native proteins, synthetic models using organic molecules, and biosynthetic models using proteins as ligands-with a greater emphasis on biosynthetic models of Cu(A), especially on new, deeper insights gained from their studies.