The concept of Yin and Yang originates in ancient Chinese philosophy and metaphysics, which describes two primal opposing but complementary forces found in all things in the universe. Yin, the darker element, is passive, dark, feminine, downward-seeking, and corresponds to the night; Yang, the brighter element, is active, light, masculine, upward-seeking and corresponds to the day.

The pair probably goes back to ancient agrarian religion; it exists in Confucianism, and it is prominent in Taoism. Though the words Yin and Yang only appear once in the Daode Jing, the book is laden with examples and clarifications of the concept of mutual arising.

Yin and Yang are descriptions of complementary opposites rather than absolutes. Any Yin/Yang dichotomy can be seen as its opposite when viewed from another perspective. The categorization is seen as one of convenience. Most forces in nature can be broken down into their respective Yin and Yang states, and the two are usually in movement rather than held in absolute stasis.

The meaning of the characters for Yin and Yang, necessarily, has more than just one connotation. Because Yang means "sunny", it corresponds to the day and more active functions. Whereas Yin, meaning "shady", corresponds to night and less active functions. Yin and Yang can be compared in the chart to the right.

It is also possible to look at Yin and Yang with respect to the flow of time. Noon, is full Yang, sunset is Yang turning to Yin; midnight is full Yin and sunrise is Yin turning to Yang. This flow of time can also be expressed in seasonal changes and directions. South and summer are full Yang; west and autumn are Yang turning to Yin; north and winter are full Yin, and east and spring are Yin turning over to Yang.

Yin and Yang can also be seen as a process of transformation which describes the changes between the phases of a cycle. For example, cold water (Yin) can be boiled and eventually turn into steam (Yang).

One way to write the symbols for Yin and Yang are a solid line (Yin) and a broken line (Yang) which could be divided into the four stages of Yin and Yang and further divided into the eight trigrams (these trigrams are used on the South Korean flag). Yin and Yang are equally important, unlike the typical dualism of good and evil.