What can emission lines tell us about an astrophysical object? This book answers that question for a host of objects, including supernovae and active galactic nuclei, across a broad range of wavelengths. The editors present sixteen review articles from internationally renowned experts in a coherent overview of the latest data, techniques and applications of the study of emission lines. Subjects include the theory of radiative transfer, shocks, photoionization, and expanding atmospheres, as well as Doppler tomography, X-ray plasmas, IR and UV spectroscopy, molecular diagnostics, spectropolarimetry and gamma-ray lines. Together these review articles provide a unique and up-to-date overview of the analysis of emission lines. In this way, they provide an excellent introduction and reference for graduate students and professionals in astronomy and physics.