'''Biofouling''' or '''biological fouling''' is the undesirable accumulation of microorganisms, plants, algae, and animals on submerged structures, especially ships' hulls. Biofouling also occurs on the surfaces of living marine organisms, when it is known as epibiosis. Biofouling is also found in membrane systems, such as membrane bioreactors and reverse osmosis spiral wound membranes. In the same manner it is found as fouling in cooling water cycles of large industrial equipments and power stations.

'''Anti-fouling''' is the process of removing the accumulation, or preventing its accumulation.

Biofouling is divided into '''microfouling''' — biofilm formation and bacterial adhesion —  and '''macrofouling''' — attachment of larger organisms, of which the main culprits are barnacles, mussels, polychaete worms, bryozoans, and seaweed.  Together, these organisms form a fouling community.

Individually small, accumulated biofoulers can form enormous masses that severely diminish ships' maneuverability and carrying capacity. Fouling causes huge material and economic costs in maintenance of shipping industries, naval vessels, and seawater pipelines. Governments and industry spend more than US$ 5.7 billion annually to prevent and control marine biofouling.

In order to minimize the impacts of foulers, many underwater structures are protected by antifouling coatings. Coatings, however, have been found to be toxic to marine organisms. For example, extremely low concentrations of tributyltin moiety (TBT), the mostly commonly used  anti-fouling agent, cause defective shell growth in the oyster ''Crassostrea gigas'' (at a concentration of 20 ng/l) and development of male characteristics in female genitalia in the dog whelk ''Nucella lapillus'' (where gonocharacteristic change is initiated at 1 ng/l). The ban of organotins such as TBT and triphenyltin (TPT), and other toxic biocides in marine coatings is a severe problem for the shipping industry; it presents a major challenge for the producers of coatings to develop alternative technologies to prevent fouling on ship hulls. Safer methods of biofouling control are actively researched. Copper and derivative compounds have successfully been used either in paints or as metal sheeting (for example Muntz metal which was specifically made for this purpose), though there is still debate as to the safety of copper<ref>[http://www.sail-world.com/index.cfm?Nid=28929]Sailing World article</ref>.

thumb|[[Коррозия ускоренная обрастанием]]
'''Обрастание''' — это поселение организмов бактерий, водорослей, беспозвоночных животных на искусственной (дно корабля, нефтяная платформа ) и естественной (любой живой организм) поверхности.

Поселение обрастателей на поверхности другого организма, называется эпибиозом. По размерам организмы обрастания делят на микрообрастателей (бактерии, одноклеточные организмы) и макрообрастателей (гидротехнических объектов, обрастатели причинают существенный экономический ущерб исчисляемых в маштабах нашей планеты миллиардами долларов мидия съедобная, гребешок и др. являются ценными промысловыми объектами. Частным случаем микрообрастания, является формирование бактериальной пленки на кариесу.