Biosorption is a physicochemical process that involves the passive and metabolically independent uptakes of pollutant by biomass. The performance of biosorption is controlled by various factors such as biosorbent selection, modification method, process parameters, and biosorption mechanisms. The biomass used most commonly as biosorbent include fungus, bacteria, yeast, and agricultural and industrial wastes. The selected biomass can be modified to alter the physicochemical properties in order to develop it as an effective biosorbent. The relationship between biosorbent and biosorbate is described by equilibrium isotherms and kinetic models. Most of the reported biosorption mechanisms include electrostatic interaction, chemisorption, and hydrogen bonding. Currently, biosorption is considered a potential step in water purification, with its relatively simple operation, high efficiency, low cost, and environmental safety. Chapter 7 describes the fundamental concepts of biosorption, types and selection of biosorbent, biosorption modeling, biosorption mechanisms, as well as the future prospect of biosorption technology.