The chapter focuses on biological treatment approaches for the removal of arsenic from contaminated environments, such as soil and water, highlighting their great potential in the context of current treatment technologies. These methods depend on microbial activity to mobilize or immobilize arsenic through processes such as sorption, biomethylation–demethylation, complexation, coprecipitation, and reduction of oxidation. Ex situ bioleaching is identified as a dependable approach for removing large amounts of arsenic from polluted soils, utilizing biostimulation techniques like the addition of carbon sources and nutrients to enhance arsenic leaching rates. Biosorption can be applied ex situ and in-situ to remove arsenic from groundwater either through sorption with biomass or coprecipitation with biogenically precipitated solid products, such as sulfurides. In situ, the effectiveness of this method hinges on the addition of appropriate biosorbents or arsenic-oxidizing microbes. Phytoremediation, another key focus of the chapter, involves using hyperaccumulator plants to extract arsenic from soils and shallow groundwater. This is achieved by genetically enhancing the plants’ ability to accumulate high levels of arsenic in their tissues, a method that can also be studied and applied ex situ, along with additional microorganisms. Biological treatments are capable of addressing arsenic contamination along with co-contaminants such as ammonia, iron, and manganese in groundwater. Two important groups of bacteria in this process include ammonia oxidizers and arsenic oxidizers, which help convert arsenic(III) to more easily removable arsenic(V). In biological treatment systems, an aeration contactor is placed before a standard granular media filter, where orthophosphate is introduced as a biological nutrient. The chapter underscores that plant-based techniques are environmentally friendly and potentially cost-effective; however, additional research and enhancement are essential to fully realize their potential.