As a system of medicine, acupuncture is over 2500 years old, and may have been practiced in China in a rudimentary form 5,000, even 7,000 years ago. The oldest continuously used medical textbook is the "Huang Di Nei Jing" ("Yellow Emperor's Internal Classic"). Still relevant today, this textbook remains a valuable reference on the theory, and on acupuncture techniques that practitioners still use today. The practice of acupuncture has evolved and changed in the last 2500 years - many new techniques have been developed, and continue to be developed today.