Based on a meeting organized jointly by ITESRE/CNR and NASA/GSFC, with support from ASI, this book summarizes past discoveries and considers the science challenges of the missions dominating the first decade of the 21st century. It looks back on a successful decade of results from the ROSAT, ASCA, RXTE and BeppoSAX missions. The two main subjects of the book are x-ray binaries and AGN, and the x-ray background. X-ray binaries provide a natural laboratory...