Indispensable for the building of cosmological models areprecise observational data. To provide such data is the mainpurpose of this book. First, an analysis of recentcosmological observations using artificial satellites andlarge ground-based telescopes is given. Among these are theobservation of the spatial distribution of galaxies andclusters, the detection of peculiar velocity fields in largeregions, and the measurement of anisotropies in themicrowave background radiation. Second, the authors presenttheoretical models which best fit the given observationaldata. The book addresses graduate students and astronomersand astrophysicists.