The supplanting of radical mastectomy by simple mastectomy and then by lumpectomy plus radiation, the use of adjuvant therapy to alter the natural course of breast and colorectal cancer, the use of tamoxifen for the prevention of breast cancer, and the dramatic improvement in survival demonstrated with the use of the monoclonal antibody trastuzumab in women with HER2-positive breast cancer are all the direct results of research that has been carried out over the past 50 years by the National Surgical Adjuvant Breast and Bowel Project (NSABP). This National Cancer Institute-supported clinical cooperative trials group based in Pittsburgh, PA, currently has 200 member institutions and 700 satellite centers located throughout the United States, Canada, Puerto Rico, and Ireland. The NSABP's mandate is to conduct large randomized phase III trials to evaluate therapies designed to improve the treatment and prevention of breast and colorectal cancer. Over the past half century, the NSABP has entered more than 150,000 patients and participants into clinical studies that have changed the treatment of colorectal cancer and have revolutionized the treatment and prevention of breast cancer.