Despite the significant progress achieved since the turn of the century in the study of the cosmic microwave background (CMB), several major problems facing modern observational cosmology remain unsolved. They include proving or disproving the inflationary theory of the evolution of the Universe and its thermal history in the pre-recombination period. The power spectrum of the initial perturbations on small scales also remains unknown. A detailed study of the CMB polarization and its frequency spectrum will shed light on these pressing questions, many of which cannot be answered based on any observations other than those of the CMB. Resolving these problems, as a by-product, will provide new information on the properties of foreground radiation (dust emission, cosmic infrared background, synchrotron radiation, free—free emission, and CO lines) and gravitational lensing. Next-generation experiments aim to address the above-mentioned problems in observational cosmology.