What can emission lines tell us about an astrophysical object? A workshop at the Space Telescope Science Institute was dedicated to address just this question – for a host of objects (including planetary nebulae and active galactic nuclei) across a broad range of wavelengths (from the infrared through to gamma-rays). Sixteen review articles from internationally renowned experts are presented in this volume. They provide an edited and coherent overview of the latest technical data, techniques in and applications of the study of emission lines from a variety of objects. Chapters include the theory of radiative transfer, photoionising shocks, and emission lines from stellar winds, as well as useful summaries of abundance determinations, atomic data, and diagnostics for IR, UV, gamma-ray and molecular lines. Together these review articles provide a unique and up-to-date overview of the analysis of emission lines. They summarise current knowledge, highlight outstanding problems and provide focus for fruitful future research. In this way they provide an excellent introduction for graduate students and reference for professionals.