Indispensable for the building of cosmological models are precise observational data. To provide such data is the main purpose of this book. First, an analysis of recent cosmological observations using artificial satellites and large ground-based telescopes is given. Among these are the observation of the spatial distribution of galaxies and clusters, the detection of peculiar velocity fields in large regions, and the measurement of anisotropies in the microwave background radiation. Second, the authors present theoretical models which best fit the given observational data. The book addresses graduate students and astronomers and astrophysicists.