Yin and yang are a philosophical concept in ancient China, containing the plain view of dialectics. Yin-yang theory of traditional Chinese medicine is a theoretical system which combines ancient philosophical thoughts and medical practices. It includes two major aspects. First, yin and yang are the fundamental laws of nature, the principles of everything in creation, and the sources for all creatures to grow, develop, and change. Second, yin and yang are opposite, interdependent, mutually waxing and waning, as well as transforming. The theory runs through the entire medical field, aiming to expound the human structure, physiology, pathology, diagnosis, prevention, and treatment.