This chapter begins with a discussion of the extraterrestrial solar spectrum and its modest variability with the 11-year solar cycle. It presents moderate spectral resolution spectrometry and moderate resolution spectral irradiance models. The chapter provides a discussion of some common types of broad spectral measurements with solar applications. Many historical papers have been written describing the scattering of solar radiation by atmospheric gases and it continues to be the subject of ongoing research. Aerosols are always present in the atmosphere. They manifest themselves as the haze often noticed when looking toward the horizon. The dominant gas absorber affecting the spectral distribution of the incident solar irradiance in the solar spectrum is water vapor with several major absorption bands in the near-infrared. Broad filter measurements of the spectral distribution of sunlight can be used for calculating the efficiency of spectrally dependent photovoltaic devices.