The nature of the far side of the Moon has always been a mystery. Throughout human history, the far side of the Moon has been so near, its presence so obvious, and yet so hidden. It was only in the decade of the 1960s that we have been able to observe it, as opposed to inferring its existence. I have been fortunate to live during the period when the far side became visible and has been extensively examined by means that were not even known 50 years ago. During this time, we have learned much but of course we have asked new questions about secrets still hidden on the Moon. Some of these secrets will be revealed in the course of future exploration and robotic spacecraft missions. This book is about what we now know of the far side of the Moon, about what the current questions are, and about some possible answers. It is a companion book to The Lunar Orbiter Photographic Atlas of the Near Side of the Moon, my first book (Byrne, 2005), also published by Springer in 2005. Together, these books cover the entire Moon in photographs taken by the robotic spacecraft that have done much of their work behind the Moon, out of communi- tion with Earth for half of their orbital periods. The photographs of the near side book were taken by a single mission of the Lunar Orbiter project, Lunar Orbiter 4 that was flown in 1967.