The ancient Chinese believed that a master force coordinates and controls the fundamental activities of different organs in the body. They designated this force as vital energy, or qi. Practitioners of traditional Chinese medicine hold that two opposite forces in the body, called the yin and the yang, can be kept in balance by acupuncture, thereby promoting health and controlling disease. The yin, the female principle, is considered to be passive and dark and is represented by the earth. The yang, the male principle, is conceived of as active and light and is represented by the heavens.

The qi is believed to flow through the body along a system of 12 meridians, or channels, on which more than 500 acupuncture points are located. An imbalance of yin and yang is thought to obstruct the flow of the qi. Acupuncture is said to affect the distribution of yin and yang in the meridians, bringing them into balance so that the qi can flow freely and so bring both physical and emotional health.