  The first generations of astrophysical objects made a substantial impact on
our Universe with their radiation. X-rays from the first sources, with large
mean free paths, likely quickly heated the intergalactic medium (IGM). The
second generation of 21cm instruments can provide a unique view into this early
epoch. The early stages of reionization likely followed, driven by so-called
"minihalos", i.e. molecularly-cooled halos. These small halos were susceptible
to complex feedback mechanisms, especially from the soft-UV background which
preceded reionization, resulting in complex and possibly extended early stages
of reionization. When atomically-cooled galaxies emerged as the dominant
ionizers, reionization could proceed more rapidly, with these being less
sensitive to radiative feedback than previously thought. Reionization could
have slowed in the final stages when the ionized bubbles grew larger than the
separation of Lyman limit absorption systems (LLSs). The final stages likely
involved the photo-evaporation of LLSs, which by then regulated the rise of the
UV background. I discuss the theoretical underpinnings of this narrative, as
well as how future 21cm observations may help shed light on the outstanding
uncertainties.
