Access to safe water is becoming an immense challenge in view of polluted water and water scarcity. This necessitates the need to look further for new technologies with greater efficiency to treat water and wastewater. There is an increased interest in nanotechnology to provide solutions toward water treatment in view of their increased surface area to volume ratio. Biological methods toward nanoparticle synthesis are more economical as compared to chemical based method in view of their increased availability. This chapter provides an overview of biological based methods for nanoparticle synthesis and their applications in water treatment. Although there are a number of laboratory-based results on biological-based synthesis of nanoparticles for water treatment, there are not many commercial applications of this technology. More research on the cost benefit analysis and environmental implications of the technology need to be worked out before this technology finds extensive application commercially.