Recombination lines at radio wavelengths have been - and still are - a p- erful tool for modern astronomy. For more than 30years, they have allowed astronomers to probe the gases from which stars form. They have even been detected in the Sun. In addition, observations of these spectral lines facilitate basic research into the atom, in forms and environments that can only exist in the huge dimensions and extreme conditions of cosmic laboratories. We intend this book to serve as a tourist's guide to the world of Radio Recombination Lines. It contains three divisions: a history of their discovery, the physics of how they form and how their voyage to us in?uences their spectral pro?les, and a description of their many astronomical contributions todate.Theappendixincludessupplementarycalculationsthatmaybeuseful to some astronomers. This material also includes tables of line frequencies from 12MHz to 30THz (?=10?m) as well as FORTRAN computer code to calculate the ?ne-structure components of the lines, to evaluate radial matrix integrals, and to calculate the departure coe?cients of hydrogen in a cosmic environment. It also describes how to convert observational to astrophysical units.The textincludes extensive referencestothe literature toassistreaders who want more details. We appreciate the help of L.W. Avery, D.S. Balser, T.M. Bania, T. Bastian, J.H. Bieging, H.J.J. Blom, N.G. Bochkarev, R.L. Brown, L.A. Bureeva (Minaeva), W.B. Burton, T. Alan Clark, Z.F. Dravskikh, W.C.