A ta'anit or taynis (Biblical Hebrew תַּעֲנִית taʿaniṯ or צוֹם ṣom) is a fast in Judaism in which one abstains from all food and drink, including water.

A Jewish fast may have one or more purposes, including:

A Jewish full fast lasts from sunset to darkness the following night. There are two Jewish full fast days:

The two full fast days carry four restrictions in addition to eating and drinking – one may not wash one's body, wear leather shoes, use colognes, oils or perfumes, or have sexual relations. Yom Kippur also has all the restrictions of Shabbat, and Tisha B'Av has restrictions somewhat similar to a mourner sitting shiva.

The Halakha status of the two Jewish full fasts is that they are obligatory.

Minor fasts are observed from dawn to nightfall, without additional restrictions. There are four public minor fasts:

There are additional fasts that are practiced in some communities or by individuals, but are not universally observed like the ones listed above. Since these are dependent on local custom, it is impossible to give a comprehensive list. Nevertheless, some of the most commonly observed ones include:

During the four minor fasts a number of changes is made to the liturgy: