We have compiled a near-global catalog of impact craters on Mercury for diameters >=20 km from images obtained during the flybys of the Mariner 10 and MESSENGER spacecraft. The observed variations in crater density suggest that the smooth plains within and around the Caloris basin are the most prominent contiguous, comparatively young regions on Mercury; no other comparably large area appears as young. In more heavily cratered terrain, even the most densely cratered regions on Mercury are deficient in craters between 20 km and ˜100 km in diameter compared with the Moon, a result that extends an observation made from Mariner 10 images of a smaller fraction of Mercury's surface. This deficit is interpreted to reflect crustal resurfacing of Mercury early in its history, most likely by volcanic processes. For craters larger than ˜100 km in diameter, the density of craters on Mercury in a given size range is similar to that of the Moon. Because such larger craters are less easily removed by volcanic resurfacing, we interpret this similarity to be the result of impact saturation effects on both bodies.