This book is an introduction to the representation theory of compact Lie groups, following Hermann Weyl's original approach. Although the authors discuss all aspects of finite-dimensional Lie theory, the emphasis throughout the book is on the groups themselves. The presentation is consequently more geometric and analytic than algebraic in nature. The central results, culminating the Weyl character formula, are reached directly and quickly, and they appear in forms suitable for applications to physics and geometry. This book is a good reference and a source of explicit computations, for physicists and mathematicians. Each section is supplemented by a wide range of exercices, and geometric ideas are illustrated with the help of 24 figures.