Natural Cordyceps sinensis is a precious medicinal substance and features a broad spectrum of health benefits for "Yin-Yang" double invigoration, health maintenance, disease amelioration, post-disease recovery, and anti-aging therapy. It has a rich history of clinical use for centuries in China and Asian countries. This organism grows only in areas of high elevation on the Qinghai-Tibetan Plateau and features a complex life cycle. The Chinese Pharmacopeia defines natural C. sinensis as a complex of multiple Ophiocordyceps sinensis fungi and a dead larval body from the family Hepialidae. Studies have reported that C. sinensis comprises more than 90 intrinsic fungal species spanning more than 37 genera and at least 17 genotypes of O. sinensis fungi. The anamorph-teleomorph connection of O. sinensis has been the subject of a decades-long academic debate. This book describes mycology and molecular biology studies of C. sinensis insect-fungi complex and discusses scientific issues and hypotheses relating to the identification and grouping of O. sinensis fungi and their biological relation to the genetic heterogeneity and heterokaryotic structure of natural C. sinensis.