In boating, a fender is used to absorb the kinetic energy of a boat or vessel berthing against a jetty, quay wall or other vessel. Fenders, used on all types of vessels, from cargo ships to cruise ships, ferries and personal yachts, prevent damage to vessels and berthing structures. To do this, fenders have high energy absorption and low reaction force. Fenders are typically manufactured out of rubber, foam elastomer or plastic. Rubber fenders are either extruded or made in a mold. The type of fender that is most suitable for an application depends on many variables, including dimensions and displacement of the vessel, maximum allowable stand-off, berthing structure, tidal variations and other berth-specific conditions. The size of the fender unit is based on the berthing energy of the vessel which is related to the square of the berthing velocity.

Historically, fenders were knotted from rope in a variety of patterns and shapes. Often damaged lines would be used for this, because they could not be used to safely moor a ship or carry a load anymore. Rope fenders are still used today by historic boat owners and are still offered by a small number of sellers.

Yachts, small leisure craft and support vessels typically have mobile fenders which are placed between the boat and the dock as the boat approaches the dock. Docks and other marine structures, such as canal entrances and bases of bridges, have permanent fenders placed to avoid damage from boats. Old tires are often used as fenders in such places.

Fendering is also used on ports and berths as well. The fendering systems act as elastic buffer devices that are used to slow ships down and prevent damage to the ship or dock structure in the mooring process.