In describing some of the laws relating to a person who has become טמא, ritually impure, through having experienced a certain type of bodily discharge, the Torah states that if such a person touches somebody else, that person too becomes טמא, as do his clothes, if the original person has not washed his hands with water (ויקרא ט"ו:י"א). It is clear from the words of Rashi )שם( that the true intent of the Posuk )שם( is to explain that this person can make other people טמא through physical contact as long as he has not immersed his entire body (not just his hands) in water; Rashi )שם( then explains, citing the Beraisa in the Sifra on our Parsha )פרק זבים, פרק ד' הלכה ה'(, that the hands were singled out simply as an example of a visible part of the body, teaching that when one who is טמא immerses himself in water, only the visible parts of his body must become wet, and not the "hidden" parts (such as the inside of the mouth or the nose, and so on). This ruling is also found in the Gemara in Niddah (דף ס"ו:), and is accepted by the Rambam (פרק א' מהל' מקואות הלכה י'), who cites the Posuk in our Parsha (שם), and by the Shulchan Aruch (יורה דעה סימן קצ"ח סעיף מ"ג). The Ramban )ויקרא שם( adds that the hands are specified simply because it is one's hands that generally do the touching, but really the person's entire body must be immersed, or he will, by touching another person, make that person טמא as well. The Rambam (שם הלכה ב'), again citing this Posuk in our Parsha )שם(, likewise rules that one who is טמא must actually immerse his entire body in water, adding that if even a small part of one's body is not immersed, the entire immersion is invalid and the person remains טמא, an idea indicated by an earlier Beraisa in the Sifra (פ' שמיני, פרק ח' הלכה ח'). It is worth noting that according to the Sifra in our Parsha )שם הלכה ו'(, one who does not immerse himself properly remains טמא indefinitely; a similar idea is expressed by the Gemara in Shabbos (דף ס"ד:, ועיין שם ברש"י ד"ה תהא) regarding another type of person who is טמא, and this ruling is codified by the Rambam (פרק ד' מהל' איסורי ביאה הלכה ג', ושם פרק י"א הלכה ט"ז, ועיין בהל' מקואות שם הלכה א') and by the Shulchan Aruch )שם סימן קצ"ז סעיף א'(.
Although the above is the true meaning of the Posuk in our Parsha (שם) in the context in which it appears, the Gemara in Chulin (דף ק"ו.), which likewise indicates that a full bodily immersion, and not just the washing of one's hands, is necessary for one to become טהור, ritually pure, adds, however, that this Posuk (שם) also provides a hint for the Chachomim to the Mitzvah of Netilas Yadayim, washing one's hands. It is clear from the presentation in the Gemara both there (שם) and on the previous pages (שם דף ק"ה.-ק"ה:) that the discussion is about the Netilas Yadayim done before a meal; Rashi, commenting on a Gemara in Niddah (דף מ"ג. בד"ה לא שטף), states clearly that the phrase in the Posuk in our Parsha (שם) is teaching us about Netilas Yadayim for a meal, while Rashi in Chulin (דף ק"ו. בד"ה אלא ה"ק) explains how the Posuk (שם) may be read to imply this, although he stresses that this is all just a hint from the Torah. The earlier Gemara in Chulin (דף ק"ה.) states that [washing for] "Mayim Rishonim," which is what we call Netilas Yadayim, is a Mitzvah; Rashi (שם בד"ה מצוה) indicates that the Mitzvah is to listen to the words of the Chachomim, as expressly stated by the later Gemara in Chulin (דף ק"ו.), meaning that Netilas Yadayim is a Mitzvah MideRabbanan, as also noted by Rashi (שם בד"ה מצוה).
The Gemara (שם) also says that Netilas Yadayim was instituted in order to help allow for the proper fulfillment of a Halacha relating to the consumption of food given as Terumah to a Kohein. Rashi (שם בד"ה נט"י) explains that one's hands have the capacity to render Terumah food unfit for consumption, as stated clearly by the Mishnah in Zavim (פרק ה' משנה י"ב), and codified by the Rambam (פרק ח' מהל' אבות הטומאות הלכה ח'); this unique Halachic status of one's hands in general is presented in the Gemara in Berachos (דף נ"ב.) and earlier in Chulin (דף ל"ג.), and is also codified by the Rambam (שם הלכה ב'). This ruling about the hands is listed by the Gemara in Shabbos (דף י"ג:) as one of the eighteen enactments instituted by the Rabbanan on a certain special day; the Gemara later in Shabbos (דף י"ד.-י"ד:) elaborates a bit on the nature of this enactment, adding that the reason behind it is that the hands are "busy" (עסקניות), meaning, as Rashi (שם בד"ה עסקנית) explains, that they often touch certain things after which it would be inappropriate to handle Terumah. The Rambam (שם) states clearly that this status regarding the hands was imposed by the Rabbanan, adding (שם הלכות ח') that the reason is that the hands are busy and one may thus be unaware that they have touched something which is טמא. Because one's hands can contaminate Terumah, the Mishnah in Challah (פרק א' משנה ט') and in Bikkurim (פרק ב' משנה א') states that before handling Terumah, one must wash one's hands, and the Rambam rules accordingly (פרק י"א מהל' תרומות הלכה ז' ובהל' מקואות שם הלכה ח'); the Mishnah in Chagigah (דף י"ח:) indicates that washing the hands is all that is necessary in order to eat Terumah, as opposed to immersing the hands or the whole body (in a Mikveh or the like), as Rashi (שם בד"ה נוטלין) explains. That Mishnah in Chagigah (שם) also states, however, that one must wash one's hands for regular food as well, referring, as the Gemara (שם) explains, to one who is eating bread, before which one must wash Netilas Yadayim; according to the aforementioned Rashi in Chulin (שם), the Chachomim instituted the requirement to wash Netilas Yadayim for regular (non-Terumah) bread, even though it is not really necessary, so that Kohanim would always be accustomed to washing their hands before eating Terumah, which is really necessary, as explained above. The Raavad, commenting on the Rambam mentioned above (השגת הראב"ד להל' אבות הטומאות שם), implies that this is indeed the basis for the Mitzvah of Netilas Yadayim.
Tosafos in Chulin (שם בד"ה מצוה לשמוע דברי חכמים) indicates that Netilas Yadayim was instituted for cleanliness reasons as well; this may be related to the Gemara in Berachos (דף נ"ג:) which indicates that washing for Mayim Rishonim (which is what we call Netilas Yadayim) is a manifestation of sanctity. This latter idea is based, apparently (עיין בהגהות הגר"א שם אות א'), on a Posuk earlier in the Torah (ויקרא י"א:מ"ד); the Maharsha in Berachos (חדושי אגדות שם בד"ה והתקדשתם) explains that Netilas Yadayim provides a person with a heightened level of Kedushah and Taharah, although he also mentions the connection to Terumah presented above. Another source for the Mitzvah of Netilas Yadayim is presented in the Sefer Tanna DeBei Eliyahu (אליהו רבא ריש פרק ט"ו) which says that just as Aharon and his sons (the Kohanim) would have to wash their hands before entering into the Mishkan, as required by the Torah (שמות ל':כ'), so too all Jews should wash their hands as a demonstration of sanctity. This may relate to the idea expressed by the Gemara in Chagigah (דף כ"ז.) that without a Mizbeiach to bring about atonement, one's table effects atonement for him; just as washing was needed before approaching the Mizbeiach, it is also needed before approaching the table to eat.
It should be pointed out that according to the Gemara in Shabbos (דף י"ד:) and in Eiruvin (דף כ"א:), Shlomo HaMelech was the one who originally instituted the practice of Netilas Yadayim; the Rambam (הל' אבות הטומאות שם) states, though, that that requirement originally related only to the consumption of meat from the Korbanos, while the Mitzvah of washing for bread in other contexts was added on later, as the Gemara subsequently (שם דף ט"ו.) indicates. The Sefer HaEshkol (הל' נטילת ידים סימן ט"ז) spells this out clearly. It should also be pointed out that there are several different reasons as to why this Mitzvah is called Netilas Yadayim, apparently emphasizing the word "Netilah" (נטילה), which usually refers to an act of taking. The Rashba (שו"ת הרשב"א חלק ז' סימן תקל"ד) quotes from Rav Hai Gaon that the word נטילה suggests an act of lifting up, as in a Posuk in Yeshayahu (ס"ג:ט'), and the word is used here because one is required to lift up one's hands after washing them, as indicated by the Gemara in Sotah (דף ד:); he also quotes from some Geonim that the word נטילה relates to the cup that one uses to wash with, which holds the requisite amount of water and which was commonly known by a name similar to the word נטילה. This reason is also mentioned by the Rosh in Berachos (פרק ט' סימן כ"ג), but the Maadanei Yom Tov (שם אות ר') writes that this reason alone is insufficient, and he says that the word נטילה indeed refers to an act of taking, specifically, taking the water with which the hands are washed, an expression similar to one found in a Mishnah in Yadayim (פרק א' משנה א'); the Tosafos Yom Tov, by the same author, commenting on the Mishnah in Berachos (פרק ח' משנה ב' בד"ה נוטלין-הראשון), elaborates on this idea and also states that Netilas Yadayim means, in effect, the taking of water for the purpose of washing the hands.
It is clear from the Mishnah in Taharos (פרק ז' משנה ח') that even if one says that he knows that his hands are not טמא, he must wash them anyway before eating Terumah; the Rambam (פרק ו' מהל' ברכות הלכה א') thus rules that even if one's hands are not dirty and he knows they are not טמא, he may not eat before washing them, and the Shulchan Aruch (אורח חיים סימן קנ"ח סעיף א') also rules accordingly (ועיין שם במשנה ברורה ס"ק ה'). The Shulchan Aruch (שם סעיף ג') then states that if one is going to eat less than a Kezayis of bread, he need not wash; this position is accepted by the Taz (שם ס"ק ג') and by the Vilna Gaon (ביאור הגר"א שם בד"ה אם), among others, but the Magen Avraham (שם ס"ק ד') and others appear to disagree, holding that one must wash for even such an amount of bread, albeit without a Beracha, and the Mishnah Berurah (שם ס"ק י') thus concludes that one should follow the stricter view, as does the Kaf HaChaim (שם אות י').
It is clear from the Mishnah in Yadayim (פרק א' משנה א') that the minimum amount of water that one must pour over one's hands for Netilas Yadayim is a Revi'is, which is approximately 3-4 fluid ounces; the Rambam (שם הלכה ד' ובפרק י"א מהל' מקואות הלכה ח') and the Shulchan Aruch (שם סימן ק"ס סעיף י"ג) rule accordingly. The Gemara in Shabbos (דף ס"ב:), however, quotes that one of the Amoraim would use a lot of water when washing his hands, and felt that he was rewarded for this practice; it would appear from Rashi (שם בד"ה משא) that to use only a Revi'is of water is improper, and the Ran there (שם דף כ"ח: בדפי הרי"ף בד"ה משא) appears to agree, as does the Yam Shel Shlomo in Chulin (פרק ח' סימן ט"ז). The Raavad (שו"ת תמים דעים סימן ס"ו) writes that a Revi'is may be used for Netilas Yadayim only when not much water is available, but if plenty of water is available, it is proper to use a lot to wash the hands; the Mishnah Berurah (שם ס"ק ס"ו) rules this way as well. The aforementioned Sefer HaEshkol (שם) states clearly, however, that a Revi'is is really sufficient, and this is also the position of the Sefer Rokeiach (סימן שכ"ח) and others; the Shulchan Aruch (שם סימן קנ"ח סעיף י') rules that although the minimum amount is indeed a Revi'is, it is preferable to wash with more water, if possible. The Mishnah Berurah (שם ס"ק ל"ז) points out, though, that for several reasons one should use at least a Revi'is of water for each hand. The Gemara in Chulin (דף ק"ז.) states that the vessel used to pour the water out of, which is itself something required by the Mishnah in Yadayim (שם משנה ב'), must be large enough to hold at least a Revi'is, and the Rambam (שם הלכה י"ב) and the Shulchan Aruch (שם סימן קנ"ט סעיף א') rule accordingly. The previous Mishnah in Yadayim (שם משנה א'), among other sources, indicates that one must pour water over each hand twice; the Shulchan Aruch (שם סימן קס"ב סעיף ב') speaks about washing each hand three times, but as the Mishnah Berurah (שם ס"ק כ') explains, the first time is not part of the Mitzvah of Netilas Yadayim, but is rather just to remove dirt, and thus does not count. The Mishnah Berurah (שם ס"ק י"ט) also explains the reason behind washing each hand twice, which he implies earlier (שם סימן ק"ס ס"ק ס"ו) is the proper practice. The Gemara in Chulin (דף ק"ו.) states that one must wash one's hands up to the joint (עד הפרק). Rashi (שם בד"ה עד, נדפס בריש עמוד ב' שם) seems to understand that this means the joint in the middle of the fingers, meaning that one really has to wash only one's finger-tips; this is also the position of the aforementioned Sefer Rokeiach (שם). Rabbeinu Tam, however, quoted in Tosafos in Chulin (עמוד ב' שם בד"ה אמר), understands that this refers to the first joint, meaning where the fingers are attached to the hand, and this view is also presented by Rabbeinu Yonah in Berachos (דף מ"א.-מ"א: בדפי הרי"ף בד"ה תני). Rabbeinu Yonah there (עמוד ב' שם בד"ה אבל) then indicates that the opinion of the Rif is that one should was the entire hand; the Rosh in Chulin (פרק ח' סימן י"א, ועיין שם בדברי חמודות אות מ"ד) also presents this as the opinion of the Rif, and the Shiltei HaGiborim on the Rif in Berachos (עמוד א' שם בלשון ריא"ז, בסוף אות ב') implies that this is the Halacha. The Rambam (שם הלכה ד') writes simply that one must wash up to the joint, without clarifying exactly what he means, but the Kessef Mishneh (שם) assumes that the Rambam follows the view of the Rif. The Beis Yosef, commenting on the Tur (שם סימן קס"א בד"ה שיעור), after quoting the different opinions, concludes that it is indeed proper to wash the entire hand because it does not require a great effort, and all the opinions can thus be followed. The Shulchan Aruch (שם סעיף ד') thus rules that this is the preferred practice, and the Mishnah Berurah, in his Biur Halacha (שם בד"ה וראוי), argues forcefully that one must follow that view unless there are extenuating circumstances which make it difficult.
It should be pointed out that although the Mitzvah is to wash the hands, and the Beracha over a Mitzvah is normally recited before the Mitzvah is done, as stated by the Gemara in Pesachim (דף ז:), and codified by the Rambam (פרק א' שם הלכה ג') and the Shulchan Aruch (שם סימן כ"ה סעיף ח'), Tosafos in Pesachim (שם בד"ה על) indicates that Netilas Yadayim may be an exception because when one washes one's hands after using the bathroom, one can not yet make a Beracha (since he is still unclean), and must thus wait until after he washes; in order to remain consistent, the Beracha over Netilas Yadayim is thus always recited after the washing. This idea is also presented by Tosafos in Berachos (דף נ"א. בד"ה מעקירא), and by the Rosh there (פרק ז' סימן ל"א), and by many others. Tosafos in Pesachim (שם) adds, though, that drying the hands is also part of the Mitzvah, because the Gemara in Sotah (דף ד:) states that anyone who eats bread without drying off his hands is considered like one who has eaten bread which is טמא; Rashi (שם בד"ה כל האוכל) explains that this is because eating bread that way is disgusting, and the Rambam (פרק ו' שם הלכה כ') and the Shulchan Aruch (שם סימן קנ"ח סעיף י"ב) rule accordingly. One may therefore say the Beracha on Netilas Yadayim before drying the hands, as stated clearly by the Mordechai in Pesachim (סימן תקל"ט, דף כ"ט. בדפי הרי"ף) and others. This is the ruling accepted by the Ramo (שם סימן קנ"ח סעיף י"א), and it seems to be widely followed. It is noteworthy that the Gemara in Shabbos (דף ס"ב:) and in Sotah (דף ד:) has very negative things to say about one who is not careful about Netilas Yadayim; the Kaf HaChaim (שם סימן קס"ה אות י"ב) thus quotes from the
Shaloh that one ought to become well versed in the Halachos of Netilas Yadayim.