Cohen et al.  recently confirmed the hypothesis that the Moon was resurfaced by an intense period of impact cratering ∼3.9 Ga ago and, by inference, that the Earth also sustained bombardment. Analyses of lunar impact melts indicate that at least one of the projectiles that hit the Moon was a differentiated iron-rich core, implying the bombardment was caused by asteroids. Meteorite analyses indicate asteroids in the asteroid belt were also heavily cratered ∼3.9 Ga and that the ancient cratered highlands of Mars suffered impacts at this time. Collectively, these data suggest there was an impact cataclysm that affected the entire inner solar system, resurfacing the terrestrial planets, and that the source of the impacting debris was the asteroid belt. Comets do not appear to have been important.