Dr. Greenwald is considered central to the creation and growth of the field of cancer prevention, having developed a research framework that progresses from discovery to intervention to application in the real-world population. In the early 1980s, scientific debates lingered as to whether cancer control research was feasible. As such, “prevention” research then studied causality, etiology, and epidemiology, with nothing that involved intervening. His time in the New York Public Health Department as director of cancer control from 1968 to 1978 and at the Epidemic Intelligence Service at the Centers for Disease Control helped develop his view that while research on causality and mechanisms was important, to be “prevention” there needed to be intervention—action both to lower the occurrence of cancer and to bring down cancer risk. Dr. Greenwald led the creation of the NCI community oncology program to engage local non-academic physicians, and the implementation of the first major clinical trials of agents (tamoxifen and finasteride) to prevent breast and prostate cancer. The Prostate, Colorectal, Lung, and Ovarian Cancer Screening Trial (PLCO) was carried out under his direction, and data and samples from this cohort are used in research to this day. NCI prevention research priorities then, as now, also include biomarkers, early detection, nutrition, and symptom management.