Leslie Bernstein delayed her scientific career to raise a family and received her Ph.D. in biostatistics at age 42. She then pursued a career as a cancer epidemiologist and was the first to demonstrate that exercise lowers women’s breast cancer risk. Her research seeks to identify modifiable factors like exercise, body size, and hormone therapy use, which alter cancer risk. She has also documented how chemotherapy and hormonal therapy for breast cancer affect the development of subsequent cancers, stroke, and heart disease and how certain lifestyle habits extend survival after treatment is completed. With more than 450 publications in her 29-year career, Bernstein has won numerous national and international awards for her breast cancer research, particularly for showing how exercise lowers breast cancer risk, and for her advocacy for women in science and her creative mentoring. After spending 25 years at the University of Southern California where she held an endowed chair and served as medical school dean for faculty affairs and university vice provost for medical affairs, Bernstein has built the Division of Cancer Etiology at the City of Hope Comprehensive Cancer Center and is again dean for faculty affairs.