Visionaries have long speculated over a future in which humans understand the scientific truth about their origins, control their environment on Earth, and live successfully outside of that environment. Their speculations, however, have frequently overlooked some fundamental facts: that the universe is complex and mostly inhospitable and that life as we know it evolved in the protective shelter of an atmosphere and a constant gravitational force. The knowledge obtained by space life sciences will play a pivotal role as humankind reaches out to explore the solar system. To conduct the types of space missions contemplated by the National Aeronautics and Space Administration (NASA), information is needed concerning the existence of life beyond the Earth, the potential interactions between planets and living organisms, and the possibilities for humans to inhabit space safely and productively. Our experience in space thus far has given us a glimpse of the potential problems and rewards facing humans on future missions, particularly those of long duration. Within the United States space program, NASA life sciences are responsible for acquiring knowledge that will contribute to the human exploration of space. Programs in the involved disciplines are an integral part of NASA's current and future missions, from near Earth orbit, to human missions to the Moon and Mars. To realize their objectives, they require the development and operation of diverse ground and flight facilities and close coordination with numerous scientific and governmental organizations in the United States and abroad.