This Book provides a concise bur detailed account of modern logic's three cornerstones: the completeness of first-order logic, Gödel's Incompleteness Theorems, and Turing's analysis of computability. In addition to the central text, an appendix explains the required technical terminology and facts. The main ideas behind the three cornerstones are explained in a simple, easy-to-grasp manner, and it is possible to select among the chapters and sections so that the reader becomes familiar with these ideas, even if some technicalities are skipped or postponed. A wealth of exercises accompany a wide selection of materials, including the histories and philosophical implications of the three main premises, making it useful as a textbook for undergraduate or graduate courses focusing on any of the three main themes. The material is rigorous and detailed but keeps the main ideas in sight, and there are numerous excursions into more advanced material for curious readers to explore.