At the end of last week's parsha (שמיני) the pasuk says והתקדשתם והייתם קדושים. The gemara in berachos coments on this that והייתם קדושים refers to מים אחרונים.
In some circles today מים אחרונים has fallen out of favor. I would like to explain the reason for this.
The Gemara in Chullin (105a) requires מים אחרונים before benching because of the concern of מלח סדומית that may remain on the hands from the meal, potentially causing blindness through contact with the eye. Tosafot (both in Berachos and in Chullin) comment that the minhag in their day was not to wash מים אחרונים. They justify the minhag by saying that we no longer have מלח סדומית and therefore there is no need. Many Rishonim disagree with Tosafos based on the gemara in Berachos that links מים אחרונים to Kedusha.
The Tur and Shulchan Aruch quote the opinion that requires מים אחרונים, Tosafos's opinion is quoted as a יש אומרים. The Gra vehemently rejected the opinion of Tosofos as did the Kabbalists.
The Mishna Berura (כדרכו) is machmir that one should wash מים אחרונים.
Rav Yaakov Emden (Mor U-ketzia 181) writes that people during Talmudic times would eat without cutlery, thus necessitating hand washing before benching. Now that people do not eat directly with their hands, no obligation of מים אחרונים exists, even according to the Kabbala.
Both R' Shternbuch (Teshuvos V'Hanhagos) and R' Wosner hold that women do not need to wash מים אחרונים. They hold that chiyuv today is a chumra based on kabbala and only men accepted upon themselveves chumra's based on kabbala.
In short, the basis for not washing מים אחרונים nowadays is:
1. Tosafos which is quotes as a יש אומרים in Shulchan Aruch
2. R' Yaakov Emden that nowadays we use cutlery and therefore there is no chiyuv according to everyone
3. According to R' Wosner and R' Shternbuch nowadays it is only a chumra based on kabbala