The mixture of meat and dairy (Hebrew: בשר בחלב, romanized: basar bechalav, lit. 'meat in milk') is forbidden according to Jewish law. This dietary law, basic to kashrut, is based on two verses in the Book of Exodus, which forbid "boiling a (goat) kid in its mother's milk" and a third repetition of this prohibition in Deuteronomy.

The rabbis of the Talmud gave no reason for the prohibition. Later authorities, such as Maimonides, opined that the law was connected to a prohibition of idolatry in Judaism. Obadiah Sforno and Solomon Luntschitz, rabbinic commentators living in the late Middle Ages, both suggested that the law referred to a specific Canaanite religious practice, in which young goats were cooked in their own mothers' milk, aiming to obtain supernatural assistance to increase the yield of their flocks. More recently, a theogonous text named the birth of the gracious gods, found during the rediscovery of Ugarit, has been interpreted as saying that a Levantine ritual to ensure agricultural fertility involved the cooking of a young goat in its mother's milk, followed by the mixture being sprinkled upon the fields. Still more recent sources argue that this translation is incorrect.

Some rabbinic commentators saw the law as having an ethical aspect. Rashbam argued that using the milk of an animal to cook its offspring was inhumane, based on a principle similar to that of Shiluach haken. Chaim ibn Attar compared the practice of cooking animals in their mother's milk to the slaying of nursing infants.

The Talmudic rabbis believed that the biblical text only forbade cooking a mixture of milk and meat, but because the biblical regulation is triplicated they imposed three distinct regulations to represent it:

Jacob ben Asher, an influential medieval rabbi, said that the gematria of do not boil a kid (Hebrew: לא תבשל גדי) is identical to that of it is the prohibition of eating, cooking and deriving benefit (Hebrew: [היא] איסור אכילה ובישול והנאה :), a detail that he considered highly significant. Deriving benefit was later interpreted by medieval writers to include:

The classical rabbis only considered milk and meat cooked together biblically forbidden. Jewish writers of the Middle Ages also forbade consumption of anything merely containing the mixed tastes of milk and meat. This included, for example, meat that had been soaked in milk for an extended period. The prohibition against deriving benefit was seen as being more nuanced, with several early modern authorities (including Moses Isserles and David HaLevi Segal) said that this restriction only applied to the milk and meat of g'di, not to the much wider range of milks and meats prohibited by the rabbis; other prominent medieval rabbis, like Solomon Luria, disagreed, believing that the prohibition of deriving benefit referred to mixtures of all meats and milks.

The Book of Genesis refers to young goats by the Hebrew phrase gəḏî-‘izzîm (גדי עזים), but the prohibition against boiling a kid... only uses the term gəḏî (גדי). Rashi, one of the most prominent talmudic commentators, argued that the term gəḏî must actually have a more general meaning, including calves and lambs, in addition to young goats. Rashi also argued that the meaning of gəḏî is still narrow enough to exclude birds, all the undomesticated kosher animals (for example, chevrotains and antelope), and all of the non-kosher animals. The Talmudic writers had a similar analysis, but believed that since domesticated kosher animals (sheep, goats, and cattle) have similar meat to birds and to the non-domestic kosher land-animals, they should prohibit these latter meats too, creating a general prohibition against mixing milk and meat from any kosher animal, excepting fish.

Consumption of non-kosher animals (e.g., pigs, camels, and turtles) is prohibited in general, and questions about the status of mixtures involving their meat and milk would be somewhat academic. Nevertheless, the lack of a classical decision about milk and meat of non-kosher animals gave rise to argument in the late Middle Ages. Some, such as Yoel Sirkis and Joshua Falk, argued that mixing milk and meat from non-kosher animals should be prohibited, but others, like Shabbatai ben Meir and David HaLevi Segal, argued that, excluding the general ban on non-kosher animals, such mixtures should not be prohibited.