I will review the origin of the cosmological 21-cm signal. Containing both cosmological and astrophysical components, the 21-cm line of neutral hydrogen promises to be a very powerful probe of the early Universe. Upcoming interferometers might be able to image the tomography of reionization, when light from these early generations of stars and black holes began flowing through and eventually permeating the Universe. Without the complications of RFI and the ionosphere, a lunar array could even glimpse the dawn of the first astrophysical objects, in the pre-reionization epoch. Understanding such a wealth of observational data requires efficient simulation tools, capable of exploring the dauntingly-large parameter space and simulating very large scales. In concluding, I will present the "semi-numerical" simulation we developed for this task: 21cmFAST.