《易经》是中国最古老的文献之一，并被儒家尊为「五经」之首；一般说上古三大奇书包括《黄帝内经》、《易经》、《山海经》，但它们成书都较晚。《易经》以一套符号系统来描述状态的简易、变易、不易，表现了中国古典文化的哲学和宇宙观。它的中心思想，是以阴阳的交替变化描述世间万物。《易经》最初用于占卜，但它的影响遍及中国的哲学、宗教、医学、天文、算术、文学、音乐、艺术、军事和武术等各方面。自从17世纪开始，《易经》也被介绍到西方。在四库全书中为经部，十三经中未经秦始皇焚书之害，它是最早哲学书。

《易》原有三种版本：《连山》、《归藏》和《周易》，《连山》和《归藏》已经失传，一般所称《易经》即源于《周易》发展而来，不过1993年湖北江陵荆州镇（现荆州市郢城镇）出土的王家台秦简《易占》与辑本《归藏》相符，被认为《归藏》古文再现。从甲骨卜辞来看，《易》的核心部分来源一定很古老，商代的数字卦多为六位一组，与《易经》卦象六爻同构，甲骨卜辞有“贞我旅吉”、“利；不利”；西周甲骨卜辞有“保贞，宫，吉”、“此由亦此亡，用由逋妾”，在《易》的爻辞中都有类似文本，但甲骨文中尚未发现"卦"、"筮"、"蓍"、"或"、"国"等易占应出现的字（比如“蓍”，起卦的方法从系辞起才见到）。

The I Ching ([î tɕíŋ] or Classic of Changes, is an ancient divination text and the oldest of the Chinese classics. Possessing a history of more than two and a half millennia of commentary and interpretation, the I Ching is an influential text read throughout the world, providing inspiration to the worlds of religion, psychoanalysis, business, literature, and art. Originally a divination manual in the Western Zhou period (1000–750 BC), over the course of the Warring States period and early imperial period (500–200 BC) it was transformed into a cosmological text with a series of philosophical commentaries known as the "Ten Wings." After becoming part of the Five Classics in the 2nd century BC, the I Ching was the subject of scholarly commentary and the basis for divination practice for centuries across the Far East, and eventually took on an influential role in Western understanding of Eastern thought.