This book presents the first translation into English of the treatise The Physics of Einstein completed by the young Georges Lemaître in 1922, only six years after the publication of Albert Einstein's theory of General Relativity. It includes a historical introduction and a historical critical edition of the original treatise in French supplemented by the author's own later additions and corrections. Monsignor Georges Lemaître can be considered the founder of the "Big Bang Theory" and a visionary architect of modern Cosmology. The scientific community is only beginning to take in the full measure of the legacy of this towering figure of 20th century physics. Against the best advice of the great names of his day, the young Lemaître was convinced, solely through the study of Einstein's theory of General Relativity, that space and time must have had a beginning with a tremendous "Big Bang" from a "quantum primeval atom" that produced an ever-expanding Universe with a positive cosmological constant. But how did the young Lemaître, essentially on his own, come to grips with the physics of Einstein? A year before his ordination as a diocesan priest, the young Lemaître submitted an audacious dissertation that was to earn him Fellowships to study at Cambridge, MIT and Harvard, and launched him on a scientific path of ground-breaking discoveries. Almost a century after Lemaître's seminal publications of 1927 and 1931, this highly pedagogical treatise is still of timely interest to young minds and remains of great value from a history of science perspective. The original French manuscript as well as the recently discovered additions are preserved in the Georges Lemaître Archives at l'Université catholique de Louvain (UCLouvain) in Louvain-la-Neuve, Belgium.