The physics of infrared emission processes is examined, taking into account black body radiation, radiation from dust particles, thermal plasma emission, nonthermal plasma emission (synchrotron), line emission from gaseous nebulae, and line emission from molecular clouds. Late-type stars and mass loss are considered along with early-type stars, young clusters and embedded objects, molecular clouds, H II regions, planetary nebulae, and the galactic center region. Extragalactic sources are also discussed, giving attention to galaxies in the infrared, Seyfert and related infrared galaxies, quasars and BL Lac objects, normal galaxies, infrared galaxy searches, red-shifted galaxies, and the cosmic background radiation.