The enormous importance of the vow and its serious consequences are reflected in the fact that a whole tractate of the Talmud, consisting of 11 chapters in the Mishnah and 91 folios in the Gemara, is devoted to it, excluding the Nazirite vow, to which a separate tractate is devoted. The biblical laws of vowing are to be found in Numbers 30: 1–16. No explicit provision is made there for absolution from vows (hattarat nedarim), the Bible permitting only the voiding of a vow (hafarat nedarim) in the case of an unmarried woman by her father, and a married woman by her husband, providing he did so "in the day that he heareth." Nevertheless, the rabbis evolved an elaborate machinery for the absolution of vows, although they frankly admitted that "the rules about the absolution of vows hover in the air and have nothing to support them" (Hag. 1: 8).

Tashlikh (thou shalt cast), ceremony held near a sea or a running stream on the first day of Rosh Ha-Shanah, usually late in the afternoon. When the first day occurs on the Sabbath, the ceremony is deferred to the second day, to ensure that no prayer book be carried to the riverside on the Sabbath (Peri Megadim to Sh. Ar., OH 583:2). The term itself is derived from Micah 7:19: "Thou wilt cast all their sins into the depths of the sea." The core of the ceremony is the recitation of Micah 7:18–20. Psalms 118:5–9; 33; 130; and Isaiah 11:9 are added in some rites. Kabbalists added quotations from the Zohar and there were other variants in different communities (e.g., in Kurdistan Jews actually entered the water; in certain parts of Bulgaria the ceremony was performed on the afternoon of the Day of Atonement).

The origin of the custom, not mentioned by talmudic, geonic, or early authorities, is uncertain. There is no direct reference to the custom, however, until R. Jacob Moellin (d. 1425), in his Sefer Maharil (Warsaw ed. (1874), 38a), explains it as a reminder of the midrashic tale (Tanh. Va-Yera, 22) of Abraham's refusal to be deterred from his mission to sacrifice Isaac even after Satan had transformed himself into a brook obstructing his path. Other authorities suggest that, as fish never close their eyes, so the ceremony is symbolic of G-d's eyes, ever-open (R. Isaiah Horowitz, Shenei Luhot ha-Berit (Josefow ed. (1878), 139); or, as the fate of fish is uncertain, so is the ceremony illustrative of man's plight (cf. Eccles. 9:12; R. Moses of Przemysl, Matteh Moshe, Warsaw ed., 1876, 166). R. Moses Isserles (Torat ha-Olah, 3:56, Lemberg, 1858 ed., part 3, 48b) saw the ceremony as a tribute to the Creator, to Whose work of creation (this actually starting on Rosh Ha-Shanah) the fish were the first witnesses. Thus it was recommended that the ceremony be performed on the banks of a river where living fish are found (Magen Avraham to Sh. Ar., OH 583:2). However, when this is impossible the ceremony is performed even by a well of water as is customary in Jerusalem.

The custom of shaking the pockets of one's garments during the ceremony is popularly taken as a rite of transferring the sins to the fish, but other authorities connect it with the talmudic saying that cleanliness of garments is a sign of moral purity (see Shab. 153a). To feed the fish during the ceremony is forbidden (Maharil, loc. cit.). Oriental-Sephardi Jews have practiced the custom since the time of Isaac Luria.