Manganese (Mn) is an essential micronutrient used to create enzymes that regulate several important biochemical pathways. Besides being part of Mn superoxide dismutase, Mn is a cofactor for pyruvate carboxylase synthase that initiates gluconeogenesis. It is also incorporated into several enzymes important for collagen and cholesterol formation. Several conditions and exposures can create toxicity, known as manganism, and deficiency states. This chapter serves as a brief review of Mn in human metabolism, genetic and iatrogenic conditions and environmental exposures associated with disease states, and how to treat them. Lastly, we include reference tables on recommended daily allowance of Mn by age and gender and values for body storage and excretion of Mn.