This paper is a current status report on a project focused on understanding the formation of large impact basins on terrestrial planetary bodies. A set of preliminary two-dimensional axisymmetric numerical models of collisions of asteroids with diameters from 150 to 800 km with the Moon, Mars, and Mercury illustrates the main mechanical effects of planetary-scale impacts. The target body is modeled on a regular grid with a spatial resolution of 5–10 km. Self-gravity is included in the hydrocode. The main consequence of such an impact is a deep melt pool at the center of the basin. Model results are tentatively compared with known impact basins such as South Pole–Aitken on the Moon and Hellas on Mars.