Relativistic cosmology has in recent years become one of the most exciting and active branches of current research. In conference after conference the view is expressed that cosmology today is where particle physics was forty years ago, with major discoveries just waiting to happen. Also gravitational wave detectors, presently under construction or in the testing phase, promise to open up an entirely novel field of physics. It is to take into account such recent developments, as well as to improve the basic text, that this second edition has been undertaken. The most affected is the last part on cosmology, but there are smaller additions, corrections, and additional exercises throughout. The books basic purpose is to make relativity come alive conceptually. Hence the emphasis on the foundations and the logical subtleties rather than on the mathematics or the detailed experiments per se. Aided by some 300 exercises, the book promotes a deep understanding and the confidence to tackle any fundamental relativistic problem.