Over the last century or two, mankind has had and is continuing to have a greater effect on the planet than ever before in history, or even in the history of life. If that seems to be too sweeping a statement, one need only look at the catastrophic loss of the tropical rain forests, which is rapidly bringing to an end the long evolution of the richest part of the biosphere and the source of most of the diversity of life. Now that this feat is being accomplished with herbicides, man's destructive power has won over the resilience of the environment. While this destructive use of a new chemical is intended, the widespread dispersal of poisons and pollutants may have serious effects other than the desired ones. This paper tries to put together some thoughts about toxic substances in the environment, in the hope that this will contribute towards some appreciation of the problems and towards policies on acceptable levels of pollution.