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. (read more)