Solar radiation at visible and ultraviolet wavelengths drives the chemistry of the atmosphere, by photo-dissociating relatively stable molecules into highly reactive radical fragments. Knowledge of photo-dissociation rate coefficients (J values) is crucial to understanding the behavior of global stratospheric and tropospheric ozone, the atmospheric lifetimes of gases such as carbon monoxide, methane, and non-methane hydrocarbons, and the formation of oxidants at urban and regional scales. J values depend on molecular parameters (absorption cross sections and photo-dissociation quantum yields) that are specific to the photo-reaction of interest, and on the availability of solar radiation at any specific location in the atmosphere. Advances in computer modeling of atmospheric radiative transfer now allow rapid calculation of J values for use in photo-chemistry models, and routinely include the effects of molecular absorbers and scatterers, clouds, aerosols, and surface reflections, for any location and time of the year. However, actual atmospheric conditions needed as input to the calculation are often not available. Direct measurements of J values, while in principle preferable, are technically difficult and limited in their temporal/spatial coverage, but generally support the theoretical calculations at least under optimal conditions (e. g., cloud free skies).