The report of a major conference on the long-term biological consequences of nuclear war. Carl Sagan presents the atmospheric and climactic effects, while Paul Ehrlich summarizes the biological implications. The report concludes that even a relatively small nuclear war could trigger a "nuclear winter," producing conditions much worse than previously anticipated; smoke, soot and damage to the ozone layer would blot out sunlight, producing severe and prolonged low temperatures. The report, as well as the conference, intentionally avoided drawing any policy implications from the findings. There may have been good reasons for this, but it nevertheless limits the usefulness of the report. "Consciousness raising" may be achieved, but it would be regrettable if such an important scientific undertaking on the world's most critical problem should not have an impact upon arms control policymakers.