This volume presents the lectures of the nineteenth Canary Islands Winter School, dedicated to the Cosmic Microwave Background (CMB). This relict radiation from the very early Universe provides a fundamental tool for precision cosmology. Prestigious researchers in the field present a comprehensive overview of current knowledge of the CMB, reviewing the theoretical foundations, the main observational results and the most advanced statistical techniques used in this discipline. The lectures give coverage from the basic principles to the most recent research results, reviewing state of the art...