A complete account of the fundamental techniques of general relativity and their application to cosmology. The book includes reviews of the different cosmological models and their classification, including such topics as causality and horizons, the cosmological parameters, observational tests and constraints of cosmology, symmetries and the large scale topology of space and space-time, and the use of supernovas as cosmological indicators. The perturbations to the cosmological models are discussed throughout the volume. The cosmic microwave background is presented, with an emphasis in secondary distortions in relation to cosmological models and large scale structures. Recent results on dark matter are summarised. A general review of primordial nucleosynthesis is given. Gravitational lensing is discussed in great detail. Most contributions show a balance between theory and observation. Readership: A solid background for students and researchers intending to work in the field of theoretical and observational cosmology.