The Nei Jing, or more properly, the Huang Di Nei Jing (The Yellow Emperor' Canon of Internal Medicine) is the monumental classic which was the first documentation of Traditional Chinese Medicine. It discussed acupuncture, herbal medicine, and the philosophy centering on yin and yang that underlies Chinese medical thought.

The book supposedly came about as a result of conversations between the Yellow Emperor and his physician, Qi Bo. Although the book is credited to the Yellow Emperor, though, it was most likely written more than two thousand years after his reign, in the fourth or third century BCE. It may be viewed as an early work of modern medicine, because it viewed disease as the result of interactions between the body and the environment, instead of simply supernatural events.

Although the Nei Jing was the first written work on Chinese medicine, it summarized knowledge originating in the Neolithic period, thousands of years earlier, which had been passed orally until then. It is divided into two parts, each containing 81 chapters. The Nei Jing Su Wen discusses the philosophy of disease, anatomy, diagnosis, yin and yang, and the five elements, while the Nei Jing Ling Shu focuses on specific methods and uses of acupuncture and herbs. They were actually written in the reverse of the above order, but are ordinarily published this way, allowing the general to precede the specific.

The Nei Jing is surprising from the Western perspective, in that it focuses primarily on prevention of disease rather than treatment, correcting imbalances of the qi before disease develops, and maintaining harmony in the body rather than applying medical treatment. Doctors taught their patients to stay healthy, and were often paid (in contrast to the Hippocratic tradition) only when their patients remained healthy.

The Nei Jing forms the basis for Chinese medicine, and is still widely read and published today, more than 2000 years after its original writing. It was the summary of several thousand years of exploration of Chinese philosophy and is older than any other medical book in existence.