1 Introduction.- 1.1 The Cosmos as a Laboratory.- 1.2 Spectral Lines in Astronomy.- 1.3 The Bohr Atom.- 1.

4 Spectral Lines in Radio Astronomy.- 2 RRLs and Atomic Physics.- 2.1 The First Surprising Results: The Absence of Stark Broadening.- 2.2 The Broadening of Radio Recombination Lines.- 2.3 Intensity of Radio Recombination Lines.

- 2.4 The Range of RRL Studies.- 2.5 How Many Atomic Levels Can Exist?.- 2.6 Summary.- 3 RRLs -- Tools for Astronomers.- 3.

1 Physical Conditions in H II Regions.- 3.2 Ionized Hydrogen and Helium in the Galaxy.- 3.3 Exploration of the Cold ISM by RRLs.- 3.4 RRLs from Stars and Stellar Envelopes.- 3.

5 RRLs from Extragalactic Objects.- A Constants.- A.1 Miscellaneous Constants.- A.2 Rydberg Constants.- A.2.

1 Reduced Mass.- A.2.2 Table of Rydberg Constants.- B Tables of Line Frequencies.- B.1 Frequencies Below 100 GHz.- B.

2 Frequencies Above 100 GHz.- B.3 FORTRAN Code for Fine-Structure Frequencies.- C Supplemental Calculations.- C.1 Early Estimates of Stark Broadening.- C.2 Refinements to the Bohr Model.

- D Hydrogen Oscillator Strengths.- D.1 Population of Atomic Sublevels.- D.2 Calculation of Oscillator Strengths.- D.3 Radial Matrix Integrals Code.- E Departure Coefficients.

- E.1 FORTRAN Code for Calculating bn Values.- F Observational Units.- F.1 What Radio Telescopes Measure.- F.2 How Radio Telescopes Measure.- F.

2.1 Sources smaller than the beam size.- F.2.2 Sources larger than the beam size.- F.2.3 Antenna temperature scale.

- Author Index.- References.