The standard hot big bang model predicts that today the universe has a temperature of a few Kelvin [1]. In 1964, a background signal was discovered and found consistent with a black-body spectrum at the temperature of about 3 K [2], which was soon recognized as radiation from the primordial universe [3]. Later observations confirmed the black-body spectrum and defined the main characteristics of this radiation, such as the presence of tiny anisotropies in an otherwise extremely isotropic background (Fig. 4.1).