How large must an extraterrestrial impact be to cause a peak episode of increased extinctions of life? Impact energies ≥ 3 × 107 Mt TNT (associated with terrestrial impact craters with final diameters ≥ 100 km) seem to be required to generate significant widespread climatic effects from sub-micron dust and soot in the atmosphere, leading to a distinct extinction episode (≥ 15% extinction of marine genera). Impacts creating craters smaller than ∼100 km in final diameter (in the 106 to 107 Mt TNT range) are capable of mostly regional destruction, with minimal impact on global climate or biota. These results are supported by the fact that the ages of the four known ≥ 100-km diameter craters of the last 260 My (Popigai, Chicxulub, Morokweng, and Manicouagan) are all correlative with times of documented extinction episodes, whereas smaller craters are not. The largest crater, the 180-km diameter Chicxulub crater (a ∼108 Mt TNT event) is associated with the more severe "major" mass-extinction event (≥ 45% extinction of genera) at the end of the Cretaceous.The percent species extinctions show a significant linear relationship with final crater diameter and impact energy. The very large Chicxulub impact lies close to the predicted curve of percent extinction versus impact-crater diameter (and energy), but the low-angle of impact, an unusual composition of the target area (with thick sediments rich in carbonates, sulfates and organic material), and a large excavated transient crater, may have led to the generation of unusually large amounts of CO2, widely distributed dust, soot and sulfate aerosols, and a uniquely severe impact-related environmental disaster. Chicxulub may thus be the only large-body impact associated with a "major" mass extinction in the Phanerozoic. Target sensitivity may apply to large impacts into ocean crust having only a thin cover of organic-poor and carbonate-poor pelagic sediments, and thus even large oceanic impacts (which are still unknown) may not produce enough dust, soot and aerosols to cause environmental crises leading to global extinction peaks above background levels.