The Zhouyi 周易, also called Yijing 易經, or, shortly, Yi 易, is one of the most important Confucian classics. It has not only influenced Confucian and especially Neo-Confucian thinking but is also deeply rooted in the Daoist tradition. It is so important that the discipline of yixue 易學 "Yijing studies" came into being.

The two most common ancient methods of divination was by producing cracks on the surface of turtle shells (guibu 龜卜, see oracle bones), and stalk divination (zhanshi 占筮) with the help of milfoil stalks (shicao 蓍草). With the help of a complex calculation method numbers were produced that were transformed into two different types of lines (guaxiang 卦象) of which trigrams, and then hexagrams were composed. The book Zhouli 周禮, which describes the various state offices, speaks of three different types to handle the "changes", namely the methods lianshan 連山 "connecting mountains", guicang 歸藏 "storehouse", and the "change" method of the Zhou people. The two first methods are unknown, except for a few surviving fragments recorded in Ma Guohan's 馬國翰 collectaneum Yuhanshanfang jiyi shu 玉函山房輯佚書. Some interpreters bring forward the argument that the word zhou does not refer to the Zhou dynasty, but to a kind of "circle" that encompasses all sixty-four hexagrams.

The Yijing, as it is received, consists of two parts, the classic Zhouli and a series of comments. The classic (the actual Yijing) was originally a divination book with a divination method by which 64 signs or symbols (gua 卦) are generated and interpreted. The 64 so-called hexagrams are each composed of two trigrams. There are eight trigrams in total, the famous bagua 八卦 which are also used in geomancy as corresponding to points of the compass. The trigrams consist of three lines which can be solid (the yang or male or strong lines, yangyao 陽爻, represented by the number nine) or divided (the yin or female or weak lines, yinyao 陰爻, represented by the number six). The hexagrams are constructed from bottom to top. The hexagram Heng 恒 , for instance, is described as "start 6, second 9, third 9, fourth 9, fifth 6, top 6" (初六、九二、九三、九四、六五、上六).

Concerning the arrangement of the hexagrams, there are two traditions. The first is known from the received text, in which they are divided into two series beginning with Qian 乾 and Kun 坤. The second arrangement has been found in the Zhouyi text discovered in the early Han period 漢 (206 BCE-220 CE) tomb of Mawangdui 馬王堆 near Changsha 長沙, Hunan, where the hexagrams begin with Qian> and Fou 否 and end with Yi 益. There are also other arrangements in various interpretive texts of the Han period, like Jiao Gan's 焦贛 Jiaoshi yilin 焦氏易林 or Jing Fang's 京房 Jingshi yizhuan 京氏易傳. The change of the particular lines is due to three factors, namely a natural conversion, human influence, and the supernatural influence of luck or misfortune.

The Zhouyi, as the core part of the book. is divided into two parts, the first includes the first 30 hexagrams, the second part the 34 others. The text to each hexagram is described in four parts: an illustration of the hexagram (guaxiang 卦象), the name of it (guaming 卦名), the corresponding dictum with an explanation of its meaning (guaci 卦辭), and an explanation of each particular line of it (yaoci 爻辭). The dictum (guaci) includes direct statement about the auspicious (ji 吉), profitable (li 利), unlucky (jiu 咎) or non-auspicious (xiong 凶) character of a divination result. The guaci of the hexagram Qian are Yuan heng, li zhen 元亨，利貞。 "Originating and penetrating, advantageous and firm." (according to Legge's translation), the yaoci of the first line is Qian long, wu yong 潛龍勿用。 "The dragon lies hid in the deep. It is not time for active doing." A small part of line explanations (yaoci) is not related to divination, but includes philosophical reflections.
The commenting part, the Yizhuan 易傳, is also called the "Great commentary" Yi dazhuan 易大傳 to discern it from later commentaries by students of the Zhouyi. It consists of the following seven parts, the first three of which are divided into two parts, so that they are called the "ten wings" (shiyi 十翼). These were in ancient times believed to have been written by Confucius 孔子, an assumption that was first doubted by the Song period 宋 (960-1279) scholar Ouyang Xiu 歐陽修.

  • Tuan 彖 (tuanzhuan 彖傳) "Structure"
  • Xiang 象 (xiangzhuan 象傳) "Appearance"
  • Xici 繫辭 (xicizhuan 繫辭傳) "About the Relationship of the Hexagrams"
  • Wenyan 文言 (wenyanzhuan 文言傳) "About the characters"
  • Shuogua 說卦 (shuoguazhuan 說卦傳) "Explaining the Hexagrams"
  • Xugua 序卦 (xuguazhuan 序卦傳) "The Order of the Hexagrams"
  • Zagua 雜卦 "Miscellaneous hexagrams"

The Tuan commentary is written as an explanation of each hexagram and its lines. It includes Confucian interpretations of politics, social relations and personal cultivation. The xiang, divided into the "smaller" (xiaoxiang 小象) and the "greater appearance" (daxiang 大象) explains the wordings of the Classic, with a focus on the position of the ruler. The Xici commentary gives an overview of the position and the meaning of the Yijing in the world order and human life, and the meaning of Yin and Yang as factors creating a changing yet eternal universe. Because of its general meaning it is also called dazhuan 大傳 "Great commentary". The Wenyan commentary explains the general meaning of the first two hexagrams, Qian and Kun, which represent Heaven and Earth. The Shuogua explains how each hexagram can change into another and how this is related to the realms of Heaven, Earth and Man. It also explains with which objects the hexagrams can be identified. The Xugua is a mnemonic aid to the sequence of the hexagrams. The last commentary, the Zagua, identifies similar or opposite hexagrams and highlights their relationship to each other.

The origin of the book is uncertain. Traditionally the invention of the trigrams is acribed to the mythical ruler Fu Xi 伏羲. King Wen 周文王 (r. beginning 11th cent. BCE) of the Zhou is said to have doubled the trigrams to hexagrams and was the first to arrange them in a certain pattern or sequence. A different sequence of the hexagrams was later ascribed to Fu Xi. The ten commentaries are ascribed to Confucius. The Han period scholar Ma Rong 馬融 and the Tang period 唐 (618-907) commentator Kong Yingda 孔穎達 thought the guaci being compiled or at least developed by King Wen, the yaoci by his son, the Duke of Zhou 周公. All these statements are unbelievable, but what is sure is that different parts of book were compiled over a long period of time by different groups of persons. Early parts must have been compiled in the late Western Zhou period 西周 (11th cent. - 770 BCE), and the final evolved during the Warring States 戰國 (5th cent. - 221 BCE).

Confucian influence plays a great role, but traces of Daoist philosophy and of the contemporary correlative thinking and the Yin-Yang theory can also be found. The sequence of the commentaries largely reflect its date of composition, the Tuan being the oldest part, the Xugua and Zagua the youngest part of the commentaries.
Han period scholars have, to make studying the Zhouyi easier, divided up the Xiang and Wenyan commentaries and directly attached to the corresponding hexagrams. This is the case in Zheng Xuan's 鄭玄 and Wang Bi's 王弼 commentaries, the latter from the Three Kingdoms period 三國 (220-280). The aristocracy of the Zhou period – and also that of their predecessors, the Shang dynasty 商 (17th-11th cent. BCE) – used to prognosticate a lot on important political and social activities, like sacrifices, war, birth, voyages, marriages, or about natural disasters that affected the harvest. Statements about such events can also be found in the Zhouyi, or about internal quarrels at the court and among the nobility. Yet all these statements are expressed in a very concise way that is not easy to understand and has therefore to be explained with the help of commentaries. The hexagram Kui 睽, for example, speaks about traveling, the hexagram Bi 賁 about marriage, and the hexagram Jing 井 about the problems governing a village. The worldview of the Yijing is a bipolar one, in which Yin and Yang, the "great man" (daren 大人) and the "small man" (xiaoren 小人), fortune and misfortune, obtaining and loosing, increase and decrease, peace and stagnation, completion and lack, are opposed to each other and may in the course of time and under certain conditions transform into each other. This can also be seen in the statements to pairs hexagrams that are opposed to each other like Tai 泰, there it is said Xiao wang da lai 小往大來。 "The little gone and the great coming.", and Fou 否, which is explained with the words Da wang xiao lai 大往小來。 "The great gone and the little coming.". Such a worldview is not very far from the philosophy of the Daoist Zhuangzi 莊子 who stressed that a constant change and uncertainty befalls human life.

The text of the Zhouyi is written in very short lines or verses that often rhyme with each other. A third of the old text can therefore be called a type of poetry. The rhyme patterns are not regular but often more composed according to need, like in the hexagram Guimei 歸妹, where it is said Nü cheng kuang, wu shi; shi kui yang, wu xue 女承筐，無實；士刲羊，無血。 "The young lady bearing a basket, but without anything in it; the gentleman slaughtering the sheep, but without blood flowing from it."; or in the hexagram Dazhuang 大壯, which says Diyang chu fan, bu neng tui, bu neng sui 羝羊觸藩，不能退，不能遂。 "The ram butting against the fence, and unable neither to retreat, or to advance." The hexagram Tun 屯 includes the verse Tun ru bo ru, bia ma han ru, fei kou, hun gou 屯如，皤如，白馬翰如；匪寇，婚媾。 "Distressed and obliged to return. The horses of her chariot also seem to be retreating - not by a spoiler, but by one who seeks her to be his wife." The first two examples refer to the life of shepherds, the third one to the preparation of a marriage. They can easily be compared with the "folk songs" in the Classic Shijing 詩經 "Book of Songs". Some paragraphs in the Zhouli can be compared to the literary genre of fu 賦 "rhapsody" that found its early forerunners in descriptive texts, often of travels, as in the text to the hexagram Kui. The poetic genre of xing 興 "introducing atmosphere" (referring to the first verse) is represented by verses like Hong jian yu lu, fu zheng bu fu, fu yun bu yu 鴻漸于陸，夫征不復，婦孕不育。 "Gradually advancing into the dry plains. A husband who goes on an expedition from which he does not return, and a wife who is pregnant but will not nourish her child." of the hexagram Jian 漸, or Mingyi yu fei, chui qi yi, junzi yu xing, san ri bu shi 明夷于飛，垂其翼；君子于行，三日不食。 "Mingyi flying, but with drooping wings; When the superior man goes away, he may be for three days without eating." of the hexagram Mingyi 明夷. The genre of bi 比 "Comparison" or "Parable" is to be found in the verses Miao neng shi, bo neng lü, lü hu wei, zhi ren, xiong 眇能視，跛能履。履虎尾，咥人，凶。 "A one-eyed man can see; a lame man can walk well; one who treads on the trail of a tiger is bitten. Ill fortune for man!" (Hexagram Lü 履). From the viewpoint of linguistics, the text of the Zhouyi is very interesting for its wide use of grammatical particles, synonyms and rhyme binomes.