"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 (The Seal of the Unity of the Three). 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 the Taoist alchemical traditions.