As described in Chapters 3–6, four nickel-dependent enzymes have been isolated and characterized from various microorganisms—urease, hydrogenase, CO dehydrogenase, and methyl coenzyme M reductase. In addition, specific accessory proteins have been identified as being involved in the functional incorporation of nickel ion into urease and hydrogenase. Intracellular nickel processing functions may also be needed for nickel metallocenter assembly in CO dehydrogenase and for the synthesis of the nickel-containing coenzyme F430, a component of methyl coenzyme M reductase. These aspects of microbial nickel metabolism will not be repeated here. Rather, this chapter will focus on nickel ion transport into the microbial cell, nickel ion toxicity and resistance mechanisms in microbes, and other features related to microbial nickel metabolism.