During the first billion years of its history, our Universe transformed from its pristine primordial conditions to the galaxies and stars-filled cosmos we see around us. This period is divided into three eras: the first, known as the Universe’s Dark Ages, and refers to the period before the emergence of any astrophysical sources. The second, known as the Cosmic Dawn, is distinguished by the emergence of the first astrophysical sources of radiation (first stars) that start heating the hydrogen in the intergalactic medium. The third, known as the Epoch of Reionization, is the period in which the hydrogen ionization process, caused by the ultraviolet radiation emanating from the first generations of stars and galaxies, became the main phenomenon, lasting until the Universe is completely ionized. One of the very few probes, if not the only one, of the first billion years, is the 21 cm radiation that emanates from the diffuse atomic hydrogen that permeates the intergalactic medium at these eras. In this talk, I will review the emerging field of the so-called "21 cm cosmology" and the worldwide effort that is currently underway to observe the first billion years with radio telescopes.