Yin-Yang is a key concept in traditional Chinese medicine. In this paper I explain the relationship of Yin-Yang and the parasympathetic and sympathetic system. Our body uses both the sympathetic and the parasympathetic system to balance our body. Yin is related to the parasympathetic system, and Yang is related to the sympathetic system. In addition, there are also secondary messenger systems which match Yin-Yang. The parasympathetic system is mediated by NO-cGMP, and the sympathetic system is mediated by CO-cAMP. NO & CO are hydrophobic secondary messengers, and cGMP & cAMP are hydrophilic secondary messengers.