Kidney yin, opposite to kidney yang, is the yin fluid of the kidney and primarily involved in calming, nourishing, moistening, and restricting yang heat. Being the root of yin qi of the whole body, kidney yin could restrict and regulate the mechanism of zang-fu organs, nourish zang-fu organs, tissues and orifices, as well as restrict metabolism, regulate qi transformation, slow down the generation and distribution of essence, blood, and fluids to reduce heat generation, and finally qi concentrates and develops into essence, blood, and fluids. The kidney function of storing essence, governing reproduction, growth and development, as well as receiving qi relies on the nourishment and moistening of kidney yin. If kidney yin is abundant, zang-fu organs, tissues, and orifices could be nourished and regulated, and spirit could be kept in the interior. If kidney yin is deficient such functions as restriction, calming, and cooling will be diminished, which might bring about the hyperactivity of zang- fu organs and restlessness. Deficient heat diseases will occur.