The Cosmic Microwave Background (CMB), the radiation left over from the Big Bang, is arguably the most important topic in modern cosmology. Its theory and observation have revolutionized cosmology from an order-of-magnitude science to a precision science. This graduate textbook describes CMB physics from first principles in a detailed yet pedagogical way, assuming only that the reader has a working knowledge of General Relativity. Among the changes in this second edition are new chapters on non-Gaussianities in the CMB and on large-scale structure, and extended discussions on lensing and baryon acoustic oscillations, topics that have developed significantly in the last decade. Discussions of CMB experiments have been updated from WMAP data to the new Planck data. The CMB success story in estimating cosmological parameters is then treated in detail, conveying the beauty of the interplay of theoretical understanding and precise experimental measurements.