Shatnez (or shaatnez, [ʃaʕatˈnez]; Hebrew: שַׁעַטְנֵז‎ šaʿaṭnēzⓘ) is cloth containing both wool and linen (linsey-woolsey), which Jewish law, derived from the Torah, prohibits wearing.

The relevant biblical verses (Leviticus 19:19 and Deuteronomy 22:11) prohibit wearing wool and linen fabrics in one garment, the blending of different species of animals, and the planting together of different kinds of seeds (collectively known as kilayim).

Maimonides wrote: "the heathen priests adorned themselves with garments containing vegetable and animal materials, while they held in their hand a seal of mineral.

[citation needed] Rabbi Abraham Isaac Kook suggested a Kabbalist interpretation that as shearing wool from sheep is "intellectually ... a form of theft," morally distinct from linen, the Torah distinguishes between them to instill a sensitivity towards animal welfare for their heightened status in a future elevated universe.

[9] Rabbinic Judaism maintains that shatnez was permitted in the case of the avnet (kohen's girdle), in which fine white linen was interwoven with purple, blue, and scarlet material.

[13] Torah law forbids kil'ayim (shatnez) - "intertying" wool and linen together, with the two exceptions being garments of kohanim[further explanation needed] and tzitit.

Concerning tzitzit, chazal permit using wool and linen strings in tandem only when genuine tekhelet is available, whereas kabbalist sources take it a step further by encouraging its practice.

Shatnez is prohibited only when worn as an ordinary garment, for the protection or benefit of the body,[15] or for its warmth,[16] but not if carried on the back as a burden or as merchandise.

[21] Observant Jews in current times also follow the laws of shatnez, and newly purchased garments are checked by experts to ensure no forbidden admixtures are used.

Karaite Jews, who do not recognize the Talmud, forbid the wearing of garments made with linen and wool (and fibres from any plant and/or any animal) under any circumstances.