Rapid population growth and industrial development are leading to an increasing demand for water, which naturally, results in increasing amounts of wastewater. Wastewater treatment consists of preliminary treatment, primary treatment, secondary or biological treatment and tertiary treatment, whereby the main part of biological treatment is microorganisms, which are responsible for the degradation of pollutants in wastewater.The sludge produced during wastewater treatment is divided into primary, secondary, tertiary and activated sludge. The activated sludge consists of microorganisms and organic matter. The aeration of wastewater and the activated sludge accelerates the decomposition of organic matter. The activated sludge process is the most applicable biological wastewater treatment process and the devices used in this process are numerous and varied. This paper focuses on microorganisms which are the basic prerequisites for successful biological wastewater treatment. Activated sludge microorganisms are bacteria, protozoa, metazoa, fungi and algae. Each of these organisms plays an important role in this man-made ecosystem.