Acupuncture grew out of ancient Chinese philosophy's dualistic cosmic theory of the yin and the yang (yinyang). The yin, the female principle, is passive and dark and is represented by the earth; the yang, the male principle, is active and light and is represented by the heavens. The forces of yin and yang act in the human body as they do throughout the natural universe as a whole. Disease or physical disharmony is caused by an imbalance or undue preponderance of these two forces in the body, and the goal of Chinese medicine is to bring the yin and the yang back into balance with each other, thus restoring the person to health.

An imbalance of yin and yang results in an obstruction of the vital life force, or ch'i (qi), in the body. The fundamental energy of the ch'i flows through 12 meridians, or pathways, in the body, each in turn associated with a major visceral organ (liver, kidney, etc.) and with a functional body system. Acupuncture is designed to affect the distribution of yin and yang in these channels so that the ch'i will be enabled to flow freely and harmoniously.

The actual practice of acupuncture consists of inserting needles (needle) into any of hundreds of points located over the 12 basic meridians and over a number of specialized meridians. The needles used may be slightly arrowheaded or may have extremely fine points. The typical insertion is 3 to 10 mm (0.1 to 0.4 inch) in depth; some procedures call for insertions up to almost 25 cm (10 inches). Once inserted, a needle may be twisted, twirled, or connected to a low-voltage alternating current for the duration of its use. The physician frequently inserts needles at a considerable distance from the point on which they are to act; for example, a needle inserted into the pad of the thumb is expected to produce analgesia in the abdomen. Similarly, successive points on a specific meridian may affect widely different areas or conditions; e.g., the first six points of the yin lung meridian deal primarily with swollen joints, excessive heat in joints, bleeding of the nose, heart pains, mental depression, and inability to stretch the arms above the head. The location of the points is mastered by the use of innumerable diagrams and models.

Acupuncture appears somehow to be effective in relieving pain and is routinely used in China as an anesthetic during surgery. Western visitors have witnessed ambitious (and ordinarily painful) surgical operations carried out on fully conscious Chinese patients locally anesthetized only by acupuncture.

Several theories have been advanced to explain acupuncture's effectiveness in this regard. There is speculation that the needle insertions stimulate the body's production of such natural opiates (painkilling chemicals) as endorphins or enkephalins. Others have posited that the minor stimulation of acupuncture selectively acts on impulse transmission to the central nervous system, thus closing certain neurological “gates” and blocking the transmission of pain impulses from other parts of the body. Some Western observers studying the method have suggested that acupuncture analgesia is plainly a placebo analgesia—which does not, however, detract from its effectiveness. Chinese assertions that acupuncture can actually cure disease defy rational clinical practice and have yet to be substantiated by Western medical researchers.