Soap is a surfactant cleaning mixture used for personal or minor cleaning. It usually comes in solid moulded form, termed bars. In the developed world, synthetic detergents have superseded soap as a laundry aid.

Many soaps are mixtures of sodium or potassium salts of fatty acids which can be derived from oils or fats by reacting them with an alkali (such as sodium or potassium hydroxide) at 80°–100 °C in a process known as saponification. The fats are hydrolyzed by the base, yielding glycerol and crude soap. Historically, the alkali used was potash made from the deliberate burning of vegetation such as bracken, or from wood ashes

Scented Bath soaps and Bath Beads

The earliest known evidence of soap use are Babylonian clay cylinders dating from 2800 BC containing a soap-like substance. A formula for soap consisting of water, alkali and cassia oil was written on a Babylonian clay tablet around 2200 BC.

The Ebers papyrus (Egypt, 1550 BC) indicates that ancient Egyptians bathed regularly and combined animal and vegetable oils with alkaline salts to create a soap-like substance.

A soap factory with bars of scented soap was found in the ruins of Pompeii (79 AD).

Legend has it that soap gets its name from Mount Sapo where ancient Romans sacrificed animals. Rain would send a mix of animal tallow and wood ash down the mountain and into the clay soil on the banks of the Tiber. Eventually, women noticed that it was easier to clean clothes with this "soap".

Historically, soap was made by mixing animal fats with lye. Because of the caustic lye, this was a dangerous procedure (perhaps more dangerous than any present-day home activities) which could result in serious chemical burns or even blindness. Before commercially-produced lye was commonplace, it was produced at home for soap making from the ashes of a wood fire.

In modern times, the use of soap has become universal in industrialized nations due to a better understanding of the role of hygiene in reducing the population size of pathogenic microorganisms. Manufactured bar soaps first became available in the late nineteenth century, and advertising campaigns in Europe and the United States helped to increase popular awareness of the relationship between cleanliness and health. By the 1950s, soap had gained public acceptance as an instrument of personal hygiene.

Some individuals continue to make soap in the home. The traditional name "soaper", for a soap-maker, is still used by those who make soap as a hobby. The most popular soap-making processes today are the cold process and the melt and pour process. Some soapers also practice other processes, such as the hot process, and make special soaps such as glycerin soap.

Handmade soap differs from industrial soap in that sufficient fat, and usually more, is used to consume the alkali, and in that the glycerin is not removed. Superfatted soap is more skin-friendly than industrial soap. Often emollients such as jojoba oil or shea butter are added at trace, after most of the oils have saponified, so that they remain unreacted in the finished soap.