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 wavelenghts (from the infrared through to gamma-rays). Sixteen review articles are presented in this volume. They provide an overview of technical data, techniques in and applications of the study of emission lines from a variety of objects.