This is an introductory text on Lie groups and algebras and their roles in diverse areas of pure and applied mathematics and physics. The material is presented in a way that is at once intuitive, geometric, applications oriented, and, most of the time, mathematically rigorous. It is intended for students and researchers without an extensive background in physics, algebra, or geometry. In addition to an exposition of the fundamental machinery of the subject, there are many concrete examples that illustrate the role of Lie groups and algebras in various fields of mathematics and physics: elementary particle physics, Riemannian geometry, symmetries of differential equations, completely integrable systems, and bifurcation theory.