Yasna (/ˈjʌsnə/; Avestan: 𐬫𐬀𐬯𐬥𐬀) is the Avestan name of Zoroastrianism's principal act of worship. It is also the name of the primary liturgical collection of Avesta texts, recited during that yasna ceremony.
 The function of the yasna ceremony is, very roughly described, to strengthen the orderly spiritual and material creations of Ahura Mazda against the assault of the destructive forces of Angra Mainyu. The yasna service, that is, the recitation of the Yasna texts, culminates in the apæ zaothra, the "offering to the waters." The ceremony may also be extended by recitation of the Visperad and Vendidad texts. A normal yasna ceremony, without extensions, takes about two hours when it is recited by an experienced priest.
 The Yasna texts constitute 72 chapters altogether, composed at different times and by different authors. The middle chapters include the (linguistically) oldest texts of the Zoroastrian canon. These very ancient texts, in the very archaic and linguistically difficult Old Avestan language, include the four most sacred Zoroastrian prayers, and also 17 chapters consisting of the five Gathas, hymns that are considered to have been composed by Zoroaster himself. Several sections of the Yasna include exegetical comments. Yasna chapter and verse pointers are traditionally abbreviated with Y.
 The Avestan language word yasna literally means 'oblation' or 'worship'. The word is linguistically and functionally related to Vedic Sanskrit yajna.
 The theological function of the yasna ceremony, and the proper performance of it, is to further asha, that is, the ceremony aims to strengthen that which is right/true (one meaning of asha) in the existence/creation (another meaning of asha) of divine order (yet another meaning of asha). The Encyclopedia Iranica summarizes the aim of the yasna ceremony as "the maintenance of the cosmic integrity of the good creation of Ahura Mazdā." Zoroastrianism's cosmological/eschatological perception of the purpose of humankind is to strengthen the orderly spiritual and material creations of Mazda against the assault of the destructive forces of Angra Mainyu. In that conflict, theologically speaking, mankind's primary weapon is the yasna ceremony, which is understood to have a direct, immediate effect: "[f]ar from being a symbolic act, the proper performance of the yasna is what prevents the cosmos from falling into chaos." The culminating act of the yasna ceremony is the Ab-Zohr, the "strengthening of the waters".
 The Yasna service, that is, the recitation of the Yasna texts, culminates in the Ab-Zohr, the "offering to waters". The Yasna ceremony may be extended by recitation of the Visperad and Vendidad.
 A well-trained priest is able to recite the entire Yasna in about two hours. With extensions, it takes about an hour longer. In its normal form, the Yasna ceremony is only to be performed in the morning.
 From a literary point of view, the 72 chapters consist of two nested inner cores, and an outer envelope. The outer chapters/sections (the "envelope") are in the Younger Avestan language. The middle 27 chapters include the (linguistically) oldest texts of the Zoroastrian canon. The inner chapters/sections (excepting chapters 42.1–4,52.5–8) are in the more archaic Old Avestan language, with the four sacred formulae bracketing the innermost core. This innermost core includes the 17 chapters of the Gathas, the oldest and most sacred texts of the Zoroastrian canon.
 From a ritual point of view, the liturgy can be broken into 4 major sections, each having its own internal prelude:
 Some sections of the Yasna occur more than once. For instance, Yasna 5 is repeated as Yasna 37, and Yasna 63 consists of passages from Yasna 15.2, 66.2 and 38.3. The ability to recite the Yasna from memory is one of the prerequisites for Zoroastrian priesthood.
 Translations of the Yasna liturgy now in the public domain: