Arsenic contamination of the environment (soil and water) has several causes and is a long-standing problem in many areas of the world. The anthropogenic additions of arsenic through activities like mining and rampant use of arsenic-containing chemical fertilizers in agricultural activities greatly outweigh that of the natural flux from rock, sediments, and so on. This, in part, causes its leaching into groundwater and agricultural soils. The health hazards (including cancers of skin, lungs) associated with arsenic contamination are numerous and mostly prove fatal. This metalloid makes its way to the human body more commonly than expected through the consumption of contaminated water and food, as is reported in countries like Bangladesh, Vietnam, and Germany. In the environment, geochemical cycling of arsenic is driven both by biotic and abiotic elements. Speciation and solubility of arsenic species in soil and water greatly depend on pH and redox potential, which are in turn influenced by the complex processes of biotic origins. This chapter focuses on the microbial role in arsenic speciation and transformation in both water and soil and the molecular machineries that are essential for these processes. We also discuss various existing remediation measures and future bioremediation strategies that can be investigated based on the natural microbial mechanisms.