Worldwide the greatest effects on the health of individuals and populations results from environmental degradation and social injustice, operating in consort. This paper describes the national and global causes and health consequences of these phenomena. Causes include overpopulation, pollution, deforestation, global warming, unsustainable agricultural and fishing practices, overconsumption, maldistribution of wealth, the rise of the corporation, the Third World debt crisis, and militarization and wars. Consequences include increased poverty, overcrowding, famine, weather extremes, species loss, acute and chronic medical illnesses, war and human rights abuses, and an increasingly unstable global situation that portends Malthusian chaos and disaster. Because of their scientific training, and due to their privileged socioeconomic status, physicians are in a unique position to recognize these phenomena and to act at all levels, from interactions with their patients, to volunteerism, to service and intervention in areas of great need, to direct political activism and involvement. Specific suggestions for action are discussed.