Ovarian toxicity is a well-described side effect of traditional chemotherapy in premenopausal women receiving treatment for early stage breast cancer. However, the impact of long-term endocrine therapy on ovarian function is not established, and to our knowledge, has never been directly studied. Understanding the effects of hormone therapy on ovarian aging will help breast cancer patients of reproductive age make more informed and empowered decisions regarding their treatment. The purpose of this study is to explore the relationship between tamoxifen therapy and ovarian aging. Patients will be identified through the UCSF Cancer Registry and California Pacific Medical Center Cancer Registry and will be evaluated based on age and menopausal status. Women who read about the study from clinicaltrials.gov and contact the study coordinator will also be considered for enrollment. The age of menopause onset will be assessed through surveys and will be compared to the accepted national average age of natural menopause. Biomarkers of ovarian reserve will be assessed in premenopausal women between ages 25-45 and will be compared to those of healthy age- and ethnicity-matched premenopausal controls from an ongoing RO1- funded prospective longitudinal ovarian aging (OVA) study.