Breast density is an established physical risk factor for breast cancer, applicable to the entire female population in a pre-breast-screening program and, unlike other risk factors, was shown to be affected by an intervention treatment. High breast tissue density is associated with increased risk of breast cancer. Transillumination Breast Spectroscopy (TIBS) uses a white light to provide spectral information on breast tissue density and composition (e.g. water, lipid, hemoglobin and other biomolecules). The overall goal of the TIBS program is to develop a technique that can identify individuals at high risk for breast cancer who would benefit most from improved screening methods and a risk reducing intervention (e.g. diet and lifestyle changes) and can monitor the efficacy of the risk reducing intervention itself in an individual. In a previous study, we demonstrated a high correlation between breast density assessed by TIBS and density identified by x-ray mammography. The current study is testing TIBS ability to monitor changes in an individual's breast density over time