Cordyceps sinensis has been described as a medicine in old Chinese and Tibetan medical books. It is a rare combination of a caterpillar and a fungus and found at altitudes above 4500m. Traditional healers recommend the fungus, i.e., Yartsa gunbu or Keera jhar (C. sinensis) for all diseases either as a single drug or combined with other herbs. Details on the chemical constituents of Cordyceps sinensis and a related species, Cordyceps militaris are given in the book. The health benefits of the fungus and the conditions for large scale cultivation of this fungus in vitro are also given.