Bleach is a chemical substance that removes colour, whitens and disinfects, often by oxidation.

The bleaching process has been known for millennia, but the chemicals currently used for bleaching resulted from the work of several 18th century scientists. Chlorine is the basis for the most common bleaches: for example, the solution of sodium hypochlorite, which is so ubiquitous that most simply call it "bleach", and calcium hypochlorite, the active compound in "bleaching powder". Oxidizing bleaching agents that do not contain chlorine are usually based on peroxides such as hydrogen peroxide, sodium percarbonate and sodium perborate. While most bleaches are oxidizing agents, some are reducing agents such as sodium dithionite and sodium borohydride.