2012 Reprint of 1963 Edition. Exact facsimile of the original edition, not reproduced with Optical Recognition Software. Gelfand was a Soviet mathematician who made major contributions to many branches of mathematics, including group theory, representation theory and functional analysis. The recipient of numerous awards and honors, including the Order of Lenin and the Wolf Prize, he was a Fellow of the Royal Society and a lifelong academic, serving decades as a professor at Moscow State University and, after immigrating to the United States shortly before his 76th birthday, at the Busch Campus of New Jersey's Rutgers University. He is known for having educated and inspired generations of students through his legendary seminar at Moscow State University. This treatise is devoted to the description and detailed study of the representations of the rotation group of three dimensional space and of the Lorentz group. These groups are of fundamental importance in theoretical physics. The book is also designed for mathematicians studying the representations of Lie groups. For them the book can serve as in introduction to the general theory of representations.