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We review the major impact-associated mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary. We then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general. Our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions, but we also stress that there are many uncertainties. We conclude that impacts with energies less than about 10 Mt are a negligible hazard. For impacts with energies above 10 Mt and below about 104 Mt (i.e., impact frequencies less than one in 6 × 104 years, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively), blast damage, earthquakes, and fires should be important on a scale of 104 or 105 km², which corresponds to the area damaged in many natural disasters of recent history. However, tsunami excited by marine impacts could be more damaging, flooding a kilometer of coastal plain over entire ocean basins. In the energy range of 104–105 Mt (intervals up to 3 × 105 years, corresponding to comets and asteroids with diameters up to 850 m and 1.4 km, respectively) water vapor injections and ozone loss become significant on the global scale. In our nominal model, such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects, but if a higher fraction of pulverized rock than we think likely reaches the stratosphere, stratospheric dust (causing global cooling) would also be important in this energy range. Thus 105 Mt is a lower limit where damage might occur beyond the experience of human history. The energy range from 105 to 106 Mt (intervals up to 2 × 106 years, corresponding to comets and asteroids up to 1.8 and 3 km diameter) is transitional between regional and global effects. Stratospheric dust, sulfates released from within impacting asteroids, and soot from extensive wild-fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10. Moreover, the ejecta plumes of these impacts may produce enough NO from shock-heated air to destroy the ozone shield. Between 106 and 107 Mt (intervals up to 1.5 × 107 years, corresponding to comets and asteroids up to 4 and 6.5 km diameter), dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis. Ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km², and the resulting smoke would reduce light levels even further. At energies above 107 Mt, blast and earthquake damage reach the regional scale (106 km²). Tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world's ocean basins. Fires would be set globally. Light levels may drop so low from the smoke, dust, and sulfate as to make vision impossible. At energies approaching 109 Mt (>108 years) the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids. The Cretaceous-Tertiary impact in particular struck evaporate substrates that very likely generated a dense, widespread sulfate aerosol layer with consequent climatic effects. The combination of all of these physical effects would surely represent a devastating stress on the global biosphere.