Resistance and Aerobic Exercise for Prevention in Women With Dense Breasts (REP-D)

The goal of this study is to learn more about how exercise might lower the risk of developing breast cancer in women with dense breast tissue by studying changes that occur in breast tissue and blood as a result of participating in an exercise program. The names of the study groups in this study are: * Exercise Training Group * Waitlist Control Group

The goal of this study is to learn more about how exercise might lower the risk of developing breast cancer by studying changes that occur in breast tissue and blood as a result of participating in an exercise program. Participants will be placed into one of two study groups: Exercise Training Group vs. Waitlist Control Group. Randomization means that a participant is put into a group by chance. The research study procedures include screening for eligibility and baseline and end of study assessments which include: breast biopsies, assessment of physical activity habits, aerobic and strength testing, blood draws and questionnaires. Participants randomized to the exercise group will be expected to participate in exercise sessions throughout the study. They will also be asked to complete logs of their activity. Participants will be in this research study for up to 16 weeks. It is expected that 46 women will take part in this study. The Breast Cancer Research Foundation is supporting this research study by providing funding.