《波斯古经》（又称为《阿维斯陀》）是琐罗亚斯德教（祆教）的经典，当中最古老的部份可追溯至公元前1323年。

教义概括为神学上的一元论和哲学上的二元论。认为宇宙初期有善恶两种神，善神为阿胡拉·玛兹达，意思为智能之主、光明、生命、创造、善行、美德、秩序、真理的化身。恶神为安格拉曼钮，是黑暗、死亡、破坏、谎言、恶行的化身。最后善神击败恶神，成为世界唯一存在的主宰。

波斯古经除了记述伊朗的宗教神话、赞歌、礼仪、戒律外，还包括其民族起源、历史、民间传说、英雄史诗等内容；波斯宗教中对天使、魔鬼的描述以及其末世观念和末日审判等之说，无疑都对当时的犹太教产生了一定的影响，后来更为基督教所继承。

The Avesta /əˈvɛstə/ is the primary collection of sacred texts of the ancient Aryan religion known as Zoroastrianism, composed in the otherwise unrecorded Avestan language.

The Avesta texts fall into several different categories, arranged either by dialect, or by usage. The principal text in the liturgical group is the Yasna, which takes its name from the Yasna ceremony, Zoroastrianism's primary act of worship, and at which the Yasna text is recited. The most important portion of the Yasna texts are the five Gathas, consisting of seventeen hymns attributed to Zoroaster himself. These hymns, together with five other short Old Avestan texts that are also part of the Yasna, are in the Old (or 'Gathic') Avestan language. The remainder of the Yasna's texts are in Younger Avestan, which is not only from a later stage of the language, but also from a different geographic region.