"Lie Theory," a set of three independent, self-contained volumes, features surveys and original work by well-established researchers in key areas of semisimple Lie groups. A wide range of topics is covered, including unitary representation theory and harmonic analysis. "Lie Theory: Lie Algebras and Representations" contains J. C. Jantzen's "Nilpotent Orbits in Representation Theory," and K.-H. Neeb's "Infinite Dimensional Groups and their Representations." Both papers are comprehensive treatments of the relevant geometry of orbits in Lie algebras, or their duals, and the correspondence to representations. Ideal for graduate students and researchers, each volume of "Lie Theory" provides a broad, clearly focused examination of semisimple Lie groups and their integral importance to research in many branches of mathematics.