This book is the result of a Meeting held in L'Aquila (Italy) from the 19th to the 23rd of June 1989. The aim of the Meeting was to gather together the people actively working on the Cosmic Microwave Background radiation, both from an experimental and from a theoretical point of view. In view of the intensive current activity in this field, including ongoing (COBE) and forthcoming (RELIC II, ISO, AELITA, etc. ) space missions, a meeting fully dedicated to this important topic was timely. The meeting also celebrated the 25th anniversary of the Microwave Background discovery made in 1964 by the Nobel Prize winners A. Penzias and R. Wilson. We greatly regret that we were not able to have them at the Meeting. There is of course another person whose absence we regret, namely R. H. Dicke, who motivated a generation of experimentalists and theoreticians to open and study this new field of research. As organizers of the Meeting, we would like to express our gratitude to the people who contributed to its success. We want to thank the members of the Scientific Organizing Committee for their assistance, suggestions and encouragement, the invited speakers for their excellent presentations, and the chairmen for their help in handling the various Sessions. We would like to thank P. Palazzi for her help in secretarial work, dr. L.

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Historical Remarks.- The Impact of the CMB Discovery on Theoretical Cosmology.- Discovery of the 3°K Radiation.- Small Scale CMB Anisotropy.- Small Scale Anisotropy Measurements.- Theoretical Maps of CMB Anisotropies.- Theory and Significance of the Sunyaev-Zel'Dovich Effect.- Observations of the Sunyaev-Zel'Dovich Effect.- Large and Intermediate Scale CMB Anisotropy.- Experimental Methods for the Investigation of the Large Scale CBR Anisotropy.- Medium Scale Measurements of the Cosmic Microwave Background at 3.3 mm.- CMB Spectrum.- Observations of the CMB Spectrum.- Theoretical Implications of the CMB Spectral Distortions.- Cosmic Instability From Radiation Pressure.- Other Backgrounds.- Infrared Cosmic Background Radiation.- The Origin of the Extragalactic X-Ray Background.- Cosmic Background Radiation: Components in Other Wavelength Ranges and their Relevance to Microwave Background.- Forthcoming Experiments.- Cosmic Background Radiation: The Next 25 Years.- Future Programs on Background Astronomy.- Conclusions.- Concluding Remarks-Observations.- Concluding Remarks-Theory.