As the only known probe of the evolutionary phase between decoupling of the CMB and z~6, the hydrogen spin-flip transition promises to play a crucial role for our understanding of the universe's dark ages and of reionization. The majority of our Hubble volume should emit in 21 cm, and observations of it promise to constrain more modes for obtaining fundamental cosmological information than any other probe. In practice there are many challenges, such as the orders-of-magnitude brighter galactic foregrounds and the computational requirements associated with doing radio-interferometry at multi-meter wavelengths. I will start with a brief introduction into 21 cm science, highlighting the different stages of spin temperature fluctuations. I will review the theoretical progress made over the last few years in predicting what the signal will look like, and talk about different ways of extracting cosmological information, and forecasts for the near future. I will focus on analytic and numerical modeling on the Epoch of Reionization (EoR) and discuss some of the unique statistical features imprinted into the 21 cm background by extended reionized regions.