Integrated Fixed-film Activated Sludge (IFAS) systems is a hybrid system consisting of both suspended and fixed biomass growth, and it is achieved by incorporating a media that support biofilms in existing aeration basin. By promoting the growth of both carbonaceous-removing and nitrifying bacteria, an integrated fixed-film activated sludge (IFAS) process can improve effluent quality without expanding the plant significantly. The fixed biomass, which grows on the captive support media, provides a higher biomass inventory without significantly increasing the suspended solids inventory. This significantly increases the biological capacity of the system without increasing the physical aeration basin size. The demonstration of the IFAS technology has shown that the installation of fixed film media within an existing aeration basin can provide substantial improvements in effluent quality and process operations. This paper discusses several popular IFAS systems and presents case studies to illustrate the application of the technology.