This is the twelfth in a series of conferences on environmental toxicity sponsored by the Department of Radiation Biology and Bio­ physics. The topics selected are intended to be of timely interest with a focus somewhat different from those with with similar titles. I would like to share with you some of the thought which went into the development of this year's program. Everyone here is aware of the great amount of publicity being given to some phase of this year's topic of "Polluted Rain", but I wonder how many of the younger generation realize how the entire world got into this kind of predicament. I can remember as a young­ ster that precipitation from the sky in the form of rain or snow was considered one of the ultimates in purity. That was in the era when a small number of automobiles were in use and the airplane was more of a curiosity in the developmental stage than anything of commer­ cial significance. Many homes used iceboxes for refrigeration, and summer cooling was accomplished by electric fans by the more afflu­ ent families.