Cosmological observations have ushered in a new era of precision cosmology that has allowed us to measure the geometry, composition, and age of the universe with great accuracy. An earlier epoch of accelerated expansion, inflation not only explains the large-scale homogeneity and isotropy of the Universe, but also provides a causal mechanism that results in the seeds for the subsequent growth of structure. However, what happens after inflation remains poorly understood. The end of inflation must provide a hot Big Bang, also known as reheating, which eventually must lead to a thermal bath of Standard Model particles, dark matter, and any additional Beyond the Standard Model (BSM) sectors, at least by the time of nucleosynthesis. Little is known about the history of the universe prior to nucleosynthesis, yet it is precisely during this period that a rich amount of phenomenology is anticipated from the physics of electroweak symmetry breaking and BSM physics -- currently being probed by experiments. This program will include extensive discussions of BSM models that can address these questions, as well as their experimental and observational signatures. Important topics will include dark matter and dark sectors, reheating and thermalization of the universe, and alternative cosmic histories after inflation.