Caulking is one of several different processes to seal joints or seams in various structures and some types of piping. The oldest form of caulking is used to make the seams in wooden boats or ships watertight, by driving fibrous materials into the wedge-shaped seams between planks. A related process was formerly employed to join sections of cast iron sewerage pipe.

Caulking is also the term to describe the process used to make riveted iron or steel ships and boilers watertight or steamtight.

The same term also refers to the application of flexible sealing compounds to close up gaps in buildings and other structures against water, air, dust, insects, or as a component in firestopping. In the tunnelling industry, caulking refers to the sealing of joints in segmental precast concrete tunnels, commonly by using concrete.

Traditional caulking (also spelled calking) on wooden vessels uses fibers of cotton, and oakum (hemp fiber soaked in pine tar). These fibers are driven into the wedge-shaped seam between planks, with a caulking mallet and a broad chisel-like tool called a caulking iron. The caulking is then covered over with a putty, in the case of hull seams, or else in deck seams with melted pine pitch, in a process referred to as paying.