The formation of the first galaxies and their role in reionizing the Universe remain some of the most compelling topics of modern cosmology. I will revisit the role of X-rays during these epochs, exploring their observational signatures with a powerful new simulation tool. Although difficult to isolate during cosmic reionization, X-rays likely dominate the early heating of the intergalactic medium (IGM). Therefore the cosmic 21cm signal, which is sensitive to the thermal and ionization history of the IGM, is an ideal probe of X-rays during the first billion years of our Universe.