Chinese herbal medicine is one of the great herbal systems of the world, with an unbroken tradition going back to the 3rd century BC. Yet throughout its history, it has continually developed in response to changing clinical conditions, and has been sustained by research into every aspect of its use. This process continues today with the development of modern medical diagnostic techniques and knowledge. In California, acupuncturists are required to be trained and tested for competency in prescribing herbal medicine.   Because of its systematic approach and clinical effectiveness, it has had a very great influence on the theory and practice of medicine in the East for centuries, and more recently has grown rapidly in popularity in the West. It still forms a major part of healthcare provision in China, and is provided in state hospitals alongside western medicine.