Earth-oriented satellite technology provides new opportunities to delve more effectively and more deeply than previously possible into some aspects of the behavior, physiology, and ecology of free-roaming animals in their natural ecosystems, especially in remote areas. These opportunities come at a time when an urgent need exists for accurate information about the migratory patterns of endangered species of whales; the movements and habits of polar bears throughout the Arctic; migrations of caribou in regions of projected oil pipelines; the migratory behavior and island-finding system in commercially important species of large sea turtles; the relationships of African elephants and plains game to changes in vegetation induced by subsistence cultivation, new fire regimes, and overgrazing by domestic livestock; and similar problems affecting the future of the total world ecosystem. Investigations along these lines are now feasible; and with miniaturization and new systems concepts for reducing power requirements, ultralightweight instrument packages for tracking migrating birds, even small ones, by satellite may soon become a reality. The capabilities of existing satellite systems and some applications in biological research are considered in the present paper.