The single and synergistic effects of man-made changes to the environment, such as habitat destruction, climate change, introduction of novel, long-lived chemicals into the environment, transport of exotic species and pathogens into new geographical areas, and other factors are predicted to cause widespread population declines and species extinctions of plants and animals in this century. From its inception, physiology has dealt with organismal capacities to deal with environmental change. This essay argues that physiologists, their methods and concepts can make more substantial contributions to Conservation Biology than they have to date. A few of the many ways in which physiologists can participate in Conservation Biology include formulating standards for proof of cause-and-effect relations and providing information about how environmental change could affect organismal energetics, host-pathogen relations, immune defenses, and others.