Earth sciences have played a major role in site selection, prediction and evaluation of effects, and in safeguarding the public in the nuclear test program since 1956, before the first underground nuclear explosion. Additions to the body of geologic and hydrologic knowledge have also been very great. Initially, the character of the Nevada Test Site was known only in reconnaissance fashion. Also, no one knew how to apply geology to the proposed underground test program. Ten years later, after expenditure of more than $11 million, more than 400 man years of study, many miles of exploratory drilling, and scores of successful tests, much more is known about a large portion of southern Nevada and about how to apply our knowledge to nuclear testing. Excellent base maps are now available. The geology of the Nevada Test Site and its environs is shown on 1:24,000 maps that cover 2000 sq miles; nearly 3000 sq miles of adjacent land are geologically mapped at 1:125,000 or larger scales. The complex volcanic and sedimentary stratigraphy, structure, and geologic history of this part of the Basin and Range Province have been worked out in detail. The deep fills in intermontane basins, and the ground water in them, are known in three dimensions. This accumulated knowledge results from a well-financed, well-supported approach by an integrated team of geologists, geophysicists, hydrologists and laboratory experts, working closely with engineers and nuclear physicists. The products of this symbiosis have been good for the sponsor — the Atomic Energy Commission — and for the earth sciences.