“… on the practical side, we have to solve the very urgent problems put by the deterioration of our environment. I have mentioned these points not in order to make predictions about the future, but In order to emphasize that we need not invent our problems.” Professor Werner Heisenberg, a Nobel laureate in physics in 1932, is Director of the Institute for Physics, Max- Planck-institute for Physics and Astrophysics, Munich, West Germany. He gave this lecture at the Smithsonian Institution, Washington, D.C., April 24, 1973. The lecture is reprinted here in full with the permission of Professor Heisenberg and the Smithsonian Institution Press.