What processes fixed the designs etched on the cosmic background radiation (CBR)? And what can they tell us about the early Universe, and the origin and evolution of cosmic structure? This review covers all aspects of three decades of study of this ghostly remnant of the hot Big Bang origin of the Universe, and examines the consequences for astrophysics, cosmology and theories of the evolution of large-scale cosmic structure. The observational techniques used to measure the spectrum of CBR, and its angular distribution on the sky, are examined in clear but critical detail: from the work of Penzias and Wilson in 1964, to results from NASA's Cosmic Microwave Background Explorer (COBE) satellite. This review takes these observations and shows how they have shaped our understanding of the early history of the Universe, and of the origin and evolution of the large-scale structures in it. As a reference this book is suitable for researchers, with introductory chapters in cosmology and radio astronomy provided for graduates in physics and astronomy entering into cosmology or CBR research.