Arsenic is widely distributed in nature and its pollution is an important issue of current public health. Although arsenic is toxic to organisms, microbes have evolved biotransformation mechanisms to live with arsenic, including gaining energy for growth from the redox of arsenic. Dissimilatory arsenate-respiring prokaryotes (DARPs) can reduce As(V) to As(III), chemoautotrophic arsenite oxidizers (CAOs) and heterotrophic arsenite oxidizers (HAOs) can oxidize As(III) to As(V). These microbes are phylogenetically diverse and remarkable in their arsenic metabolic diversity. They take part in the key steps of arsenic biogeochemical cycles, have potential impact in speciation and mobilization of arsenic in nature, and are involved in a global arsenic geocycle. The chemical speciation of arsenic in the stratified water column in Mono Lake may be explained by microbial arsenic cycling by tight coupling between CAOs and DARPs. In Bengal delta plain subsurface aquifers, these microbial reactions may mobilize arsenic from the solid phase into the aqueous phase, resulting in contaminated underground water.