Reprinted from The Jewish Religion: A Companion, published by Oxford University Press.
In Temple times there were elaborate rules in connection with ritual impurity. If a person had been rendered impure through having come into contact, say, with a dead rodent, he contaminated sacred food such as the tithe given to the priests, which must then not be eaten. The way in which contamination of this kind could be removed was through immersion in a ritual bath.
But the sages imposed in certain circumstances the minor form of contamination known as "hand contamination" in which only the hands, not the whole body, was contaminated and for this to be removed total immersion was not required, only the ritual washing of the hands. Since there was a good deal of priests' tithe in ancient Palestine which could easily come into contact with the hands, the sages eventually ordained that the hands of every Jew, not only the hands of a priest, must be washed ritually before meals.
It has to be appreciated that this ritual washing of the hands has nothing to do with physical cleanliness. On hygienic grounds, the hands are obviously to be clean of dirt before food is eaten. Even when the hands are physically clean they are still required to be ritually washed.
Although the original reason for washing the hands no longer applies, since there is no sacred food to be eaten, the ritual was continued on the grounds that the ideal of holiness demands a special, ritualistic washing of the hands. The act of washing the hands in this sense is seen as the introduction of the holiness ideal into the mundane life of the Jew. This ritual washing is only required before a meal at which bread is eaten.
The procedure is to pour water out from a cup or glass first twice over the right hand and then twice over the left hand--care being taken that the unwashed hands do not touch the water used for the washing. The hands are then dried with a towel before partaking of the meal. A benediction is recited over the washing of the hands: "Blessed art Thou, O Lord our God, King of the universe, who has sanctified us with Thy commandments and has commanded us concerning the washing of the hands."
The reference to the command has to be understood in the context that rabbinic ordinances are also commanded by God. Observant Jews are very strict in this matter of washing the hands before meals.
The Talmud also refers to washing the hands after meals but here the reason given is that people used to eat with their hands and a certain salt added to food in those days might cause injury to the eyes if it came into contact with them. The French authorities in the Middle Ages argued that this hygienic reason no longer obtains, since this kind of salt is no longer used.
Many observant Jews follow this line of thinking and do not wash the hands after the meal, not as a ritual in any event. But many authorities introduce the holiness motif here as well, although no benediction is recited over mayyim aharonim, "afterwards water." For those who observe it, the procedure is simply to pour a little water out of a cup or glass over the fingers of the two hands.
There is a further ritual washing of the hands on rising from sleep. This is a later innovation for which two reasons are given. One is that during sleep an unclean spirit rests on the body. This departs on waking, except for a residue left on the fingernails and to remove this, the hands have to be washed.
The second reason (perhaps introduced as a rationalization) is that a Jew, a member of the "kingdom of priests" (Exodus 19:5), must, when he rises from his bed to serve his Maker, follow the practice of the priests in the Temple who would wash their hands from the hand-basin (Exodus 30:17-21).
The procedure for this washing of the hands is to pour the water first on the right hand and then the left and to repeat this three times. Some of the more scrupulous have a cup of water and a basin at the bedside so as to wash the hands immediately on waking. Following the first reason, they will pour out the "nail water" (neggel wasser in Yiddish) and not allow it to come into contact with food or drink.
Many pious Jews also carry out the ritual of washing the hands before performing any religious act, especially before prayer. It is also the custom to perform the ritual of washing the hands on returning from the graveside after a burial.