Introduction

The phrase “infrared emission lines,” like so many other topics covered at this meeting, is extremely broad. Therefore I will begin by defining and limiting the scope of this review. I will discuss only lines that arise from gas-phase atoms, ions, and molecules, and will not include spectral features produced by interstellar dust. In this review, “infrared” will mean the spectral region 1–200 μm, which corresponds to certain types of astronomical detectors; shorter wavelengths will be called “far red” (with apologies to Don Osterbrock), and longer wavelengths, “submillimeter.” Another boundary condition is that I will restrict myself to low densities, n ≤ 108 cm–3. Apart from these constraints, I will attempt to be as general as possible, though with no pretentions to completeness. This review is organized by physical properties and methods of analysis, rather than by class of astronomical object.