Tap water (running water) is part of indoor plumbing, which became available in the developed world in the late 19th century and common in the mid-20th century.

The provision of tap water is a massive infrastructure of piping, pumps, and water purification works. The direct cost of the tap water alone, however, is a small fraction of that of bottled water, which can cost from 240 to 10,000 times as much for the same amount.

The availability of clean tap water brings major public health benefits. Usually, the same administration that provides tap water is also responsible for the removal and treatment before discharge or reclamation of wastewater.

In many areas, chemicals containing fluoride are added to the tap water in an effort to improve public dental health. In some countries, this remains a controversial issue for a portion of the population. See water fluoridation controversy.

Tap water may contain various types of natural but relatively harmless contaminants such as scaling agents like calcium carbonate in hard water and metal ions such as magnesium and iron, and odoriferous gases such as hydrogen sulfide. Local geological conditions affecting groundwater are determining factors of the presence of these substances in water.

Occasionally, there are health concerns regarding the leakage of dangerous biological or chemical contaminating agents into local water supplies when people are advised by public health officials not to drink the water, and stick to bottled water instead. An example is the recent discovery of potentially hazardous nitrates in the public water supply in Kingman, Arizona.