  This chapter describes the astrophysics encoded by the 21-cm background. We
begin with a brief introduction to the radiative transfer and ionization
chemistry relevant to the high-z intergalactic medium. Then, we will provide a
review of the most plausible sources of ionization and heating in the early
Universe. Finally, we will explore a variety of current 21-cm predictions, and
illustrate the sensitivity of the global 21-cm signal and power spectrum to
parameters of interest.
