The spin-flip of the ground-state electron in neutral hydrogen, known as hyperfine structure, gives rise to the famous 21-cm line in the radio band. The 21-cm line informs us about the “cold” phase of atomic hydrogen in the Universe. In this chapter, we present the basic physics of the 21-cm spin flip and then discuss several surveys of 21-cm absorption and their reported findings. These include blind surveys, galaxy-selected surveys, metal-line selected surveys, and DLA-selected surveys. An exciting measurement known as redshift drift, which would provide a direct measurement of the change in the expansion rate of the Universe, is expected to be highly precise using 21-cm observations. During the Epoch of Reionization, a 21-cm forest, analogous to the Ly α forest, is expected. In fact, this absorption line is expected to trace all the way back to the Dark Ages of the universe and yield empirical insights into the formation of the first stars and black holes.

The cold neutral medium (CNM) represents gas at temperature T ∼ 80 K and number density n ∼ 40 cm-−3, where heating by photoelectrons ejected from dust grains balances cooling by fine-structure line emission from C+. The cold neutral medium is studied by looking at the absorption lines caused by the CNM along the line of sight to bright background stars. Interpreting these absorption lines requires solving the equation of radiative transfer. In particular, the curve of growth for an absorption line yields the relation between the observed equivalent width of a line and the underlying column density of the atom or ion giving rise to the absorption.