Breast density is essentially the ratio of fat to fibroglandular tissue in the breast. Radiologists characterize each mammogram into one of four levels of overall density. Based on population studies, and listed in order of least dense to most dense, the frequency distribution of these categories is approximately: almost entirely fatty - 10%; scattered areas of fibroglandular density - 40%; heterogeneously dense - 40%; and extremely dense - 10%. All women who fall into the latter two categories (heterogeneously dense and extremely dense) will be informed that they have dense breasts under the law. Therefore, approximately 50% of women will fall into these two categories and be told that they have dense breasts. Currently, the determination of density by radiologists is a qualitative, visual assessment. Evolving technologies may provide more objective and quantitative density assessment in the future.