Impact cratering is a ubiquitous process on rocky bodies that may help create the conditions for the origin of life, particularly in marine environments. In settings where life is already established, however, impacts are a destructive process that sterilizes the local environment and, depending on the size of the impact, may negatively affect the entire biosphere. Understanding how pre-existing life can colonize newly formed impact craters can provide insights into impact craters as habitats, and the processes which govern the crater environment. This talk will review the evidence for initial colonization of two marine impact craters: the Cretaceous/Paleogene (K/Pg) Chicxulub crater in the Gulf of Mexico and the late Eocene Chesapeake Bay impact structure. Probably the most famous impact structure on Earth, the Chicxulub crater is mechanistically linked to the K/Pg mass extinction. Despite this severe shock on the global biosphere, drill cores from the Chicxulub crater reveal that the impact basin was inhabited by marine plankton and benthic macrofauna within years of the impact, even before deposition of the finest atmospheric ejecta. High marine productivity was established within thousands of years both within the crater and in the broader Gulf of Mexico and Caribbean region. This regional extent indicates that oceanographic processes influenced this high productivity, and not anything unique to the crater. The Chesapeake Bay impact structure is an order of magnitude smaller than the Chicxulub crater and is not associated with any global change in biodiversity, and yet the crater environment was characterized by a dead zone that persisted for thousands of years, with environmental equilibrium established by 29-190 kyr post impact in different parts of the crater. The key difference between these craters is that the Chicxulub crater rim was open to the north, allowing communication with the open Gulf of Mexico at full crater depth, while the Chesapeake Bay crater rim was unbroken, and formed a restricted basin that quickly became anoxic. It is thus crater geometry, and not impactor size, that is the primary control on the colonization of marine impact craters. This leads to the prediction that other fully enclosed marine impact craters, like the recently-discovered Nadir impact crater on the West African continental margin, were also characterized by relatively long-lived dead zones.