Stars are born and die in clouds of gas and dust, opaque to most types of radiation, but transparent in the infrared. Requiring complex detectors, space missions and cooled telescopes, infrared astronomy is the last branch of this discipline to come of age. After a very successful sky survey performed in the eighties by the IRAS satellite, the Infrared Space Observatory, in the nineties, brought spectacular advances in the understanding of the processes giving rise to powerful infrared emission by a great variety of celestial sources.