The exploration of the Universe, as conducted by physicists, astronomers, and cosmologists was one of the greatest intellectual adventures of the mid-twentieth century. This book, first published in 1971, tells the story of their achievements and the insight gained into the structure, history, working and scale of our Universe. Dr Sciama describes the major components of the Universe as understood at the beginning of the 1970s: the stars, galaxies, radio-galaxies and quasi-stellar objects. He discusses in detail the red shift of the lines in their optical spectra, which leads to the idea that the Universe is expanding. Theoretical discussion of the expanding Universe suggests the possibility that intergalactic space may contain a significant quantity of matter and be the seat of important physical activity. The issues involved are thoroughly debated. Also discussed is the discover and significance of the 3'K' cosmic microwave radiation, its relation to the hot big bang and the helium problem, to cosmic high energy processes and to questions of isotropy.