Lie groups and Lie algebras arose in the study of the mathematical properties of rotations. The ideas were originally developed by the Norwegian mathematician Sophus Lie (1849-1899), and they play an important role in the development of 20th century mathematics and mathematical physics. This book focuses on the origins and subsequent development of the structure of the theory, particularly the structure and representations of semisimple groups, with particular emphasis on the motivation behind the mathematics. It should thus be useful not only to historians of mathematics and to mathematicians interested in the history of their field, but also to students.