Cosmology is the study of nature on the grandest scale we know: that of the universe itself. This successful and class-tested textbook covers all the essentials: galaxies, their formation, morphology, dynamics, stellar and gaseous structure, and interactions; radio sources, quasars, and other galactic nuclei and their spectra and absorption-line systems; and the interstellar medium and the universe on a large scale - in short, all of extragalactic astronomy. The emphasis throughout is on data acquired at all wavelengths thanks to modern observational techniques and on the concepts relevant to each type of object. The authors present the thinking that lies at the heart of the newest theories and developments, including the formation of stars and spiral and barred structures, the origin of absorption lines, the properties of galaxies, and recent inflationary theories in cosmology. In addition, the authors have thoroughly revised and updated the text of the French original for this new translation.