Preface.- Preface to First Edition.- General Introduction.- Photoionization Equilibrium.- Thermal Equilibrium.- Calculation of Emitted Spectrum.- Comparison of Theory with Observations.- Internal Dynamics of Gaseous Nebulae.- Interstellar Dust.- Infrared Radiation and Molecules.- H II Regions in the Galactic Context.- Planetary Nebulae.- Heavy Elements and High-Energy Effects.- Nova and Supernova Remnants.- Active Galactic Nuclei Diagnostics and Physics.- Active Galactic Nuclei Results.- Appendix 1: Measures of Light.- Appendix 2: Milne Relation Between Capture and Photoionization Cross.- Appendix 3: Emission Lines of Neutral Atoms.- Appendix 4: Nebular Quantum Mechanics.- Appendix 5: Atomic Data for Heavy Element Ionization Balance.- Appendix 6: Quantum Mechanics of Molecules.- Glossary of Physical Symbols.- Glossary of Acronyms.- Index.