Biofouling is caused by the accumulation of microorganisms on solid surfaces in contact with water. The impact of biofouling is higher economical costs for preventing and controlling it because biofouling usually decreases the performance of systems or devices. Biofilm is considered as a major cause of biofouling, and it can be formed under circumstances where water is in contact with other materials. Biofilm prevention or removal methods have been required. In this chapter biofouling, the mechanism of biofilm formation, and the impacts of biofouling in some industries are introduced.