Biofouling or biological fouling is the undesirable accumulation of microorganisms, plants, algae and/or animals on wet structures.

Biofouling is particularly and economically significant on the hulls of ships, where high levels of proliferation can reduce the performance of the vessel and increase its fuel requirements.[1][2]​.

Biofouling is also found in almost all cases where water-based liquids are in contact with other materials. Industrially important examples include membrane systems, such as membrane bioreactors and reverse osmosis, spiral membranes, industrial water cooling cycles of large industrial equipment and power generation plants).

It can also occur in pipelines that carry oils as entrained water, especially those that transport used oils, cutting oils, soluble oils, or hydraulic oils.

Solutions to biofouling are essentially based on two principles, preventing organisms from adhering to surfaces in contact with water or killing said organisms to prevent them from adhering and proliferating.

On ship hulls and structures that are permanently in contact with water, the most commonly used are antifouling paints, also called patents. These paints prevent organisms from adhering to the treated surfaces.

In pipes, the systems used are based on the elimination of organisms through the use of a biocide. In pipes and conduits of ships and marine structures, cathodic protection is very commonly used to generate copper ions. Copper ions are very toxic to the organisms that cause blockages in the pipes. Once the ions are generated, they move with the water current of the pipe and kill the small organisms that seek to adhere to the internal walls of the pipe or water intake.