How well Do We Understand the Hydrogen 21 cm Line?

Hyperfine intervals in light hydrogenic atoms and ions are among the most accurately measured quantities in physics. The theory of QED corrections has recently advanced to the point that the uncertainty of these corrections for hydrogenic atoms and ions is probably smaller than 0.1 parts per million (ppm), which is nevertheless not as accurate as the experiments. One such line is the 21 cm line in hydrogen. An assessment is made of how well we understand the theory of this transition.