Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water is more compatible with soap and extends the lifetime of plumbing. Water softening is usually achieved using ion-exchange resins.

The presence of certain metal ions in water causes a variety of problems. These ions interfere with the action of soaps. They also lead to build up of limescale, which can foul plumbing, and promote galvanic corrosion.