Industrial wastewater varies in quality and quantity depending on the industry type and may contain various toxic compounds. In a biological process, organic substances in the wastewater are used as food by bacteria and other microorganisms. Several biological reactors conventionally used for industrial wastewater treatment are briefly discussed in this chapter. Biological reactors are categorized as aerobic, anaerobic, and a combination of both based on their oxygen requirement. The suspended and attached growth processes are a further subdivision of aerobic and anaerobic processes. Advanced biological reactors such as membrane bioreactors and granular sludge technologies are primarily used in industry. Industrial wastewater contains various organic and inorganic pollutants. The occurrence of toxic compounds imparts toxic or inhibitory effects on microorganisms and may lead to failure of the biological units. Thus, the impact of several industrial wastewaters on various biological reactors are also discussed in this chapter.