This book discusses the representation theory of the group SL(2, R), and some applications of this theory. The emphasis is in fact on the applications, some of which are outside representation theory and some are to representation theory itself. The topics outside representation theory are mostly of substantial classical importance (Fourier analysis, Laplace equation, Huyghen's Principle, Ergodic theory), while those inside representation theory mostly concern themes that have been central to Harish-Chandra's development of harmonic analysis on semisimple groups. This mix of topics should appeal to non-specialists in representation theory by illustrating how the theory can offer new perspectives on familiar topics, and by offering some insight into some important themes in representation theory itself.