"The Cantong qi is the forefather of the scriptures on the Elixir of all times. Its words are ancient and profound, arcane and subtle. No one can fathom their meaning." Thus begins a preface found in one of the commentaries to the Cantong qi. These words express several significant features of the work translated in the present book: the charm of its verses, the depth of its discourse, its enigmatic language, and its intimate relation to Taoist alchemy (Waidan and Neidan). Under an allusive poetical language and thick layers of images and symbols, the Cantong qi hides the exposition of a doctrine that inspired a large number of commentaries and other works, and attracted the attention not only of Taoist masters and adepts, but also of philosophers, cosmologists, poets, literati, calligraphers, philologists, and bibliophiles. Neidan (Internal Alchemy) is the legacy that has shaped the dominant image and understanding of the Cantong qi in China, by placing this work at the origins of its teachings and practices. Besides this one, there has been, within the Taoist tradition, a second, less well-known way of reading the text: the Cantong qi is concerned not with one, but with three major subjects, namely Cosmology (the system of the Book of Changes), Taoism (the way of "non-doing"), and Alchemy, and joins them to one another into a single, unique doctrine. In addition to a complete translation of the Cantong qi, this book - the fruit of more than 20 years of work - contains explanations of each of its sections, notes on many of its verses, and a detailed introduction to its history and doctrines.