Molybdenum and tungsten are the only elements of the second and third row transition series to have known biological functions. Molybdenum has been recognized since the 1930s [1] for its role in nitrogen fixing enzyme systems (see Chapter 7). However, from 1953 [2], it was realized that molybdenum is essential for diverse aspects of metabolism in a wide range of organisms. Tungsten was first identified in 1973 [3] in an enzyme from a thermophilic organism and since 1990 has been extensively studied [4,5] in the hyperthermophilic archaea (formally termed archaebacteria).