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

Inclusion Criteria:

* Women aged 18-59
* Heterogeneously dense/extremely dense (BIRADS 3 or 4) breast tissue on mammogram within the past 12 months
* Physically inactive; engaging in \<60 minutes of moderate or vigorous intensity exercise per week, as assessed by Godin Leisure Time screener
* Not pregnant or breastfeeding
* English speaking and able to read English. Investigators are only enrolling English-speaking participants to this study at this time due to the fact that investigators currently only have English-speaking exercise trainers with the proper qualifications to conduct the exercise training
* Written informed consent prior to any study-related procedures

Exclusion Criteria:

* Self-reported inability to walk 2 blocks (at any pace)
* Prior history of breast cancer; prior DCIS is allowable as long as participant is not taking endocrine therapy and has at least 1 breast that has not been irradiated
* On oral or implantable hormonal contraceptives, postmenopausal hormone replacement therapy, a selective estrogen receptor modulator or an aromatase inhibitor; Mirena or other IUD is acceptable
* Consistent strength training in the past 3 months
* Use of weightloss drugs
* Cardiovascular, respiratory or musculoskeletal disease or joint problems that preclude moderate exercise. Examples would include unstable angina, recent myocardial infarction, oxygen-dependent pulmonary disease, and osteoarthritis requiring imminent joint replacement. Moderate arthritis that does not preclude physical activity is not a reason for ineligibility
* Psychiatric disorders or conditions that would preclude participation in the study intervention (e.g. untreated major depression or psychosis, substance abuse, severe personality disorder)