A review is made of the current state of knowledge about the iron-sulphur proteins. The physicochemical and structural characteristics of these proteins are described and the possible information that examination of these proteins may yield on different aspects of chemical evolution is discussed. These proteins are required for numerous enzymatic reactions dependent on electron transfer. Discussion is made of hypotheses that different iron-sulphur proteins may function in regulating enzymatic reactions. The similarities and differences between adrenodoxin and other iron-sulphur proteins are discussed.