Breast density refers to the proportion of different types of tissue in a woman's breasts, including fatty tissue and glandular tissue. Dense breasts have a higher amount of glandular and connective tissue relative to fatty tissue, while non-dense or low-density breasts have more fatty tissue. Breast density is an important factor in breast cancer screening and risk assessment, as dense breasts can make it more difficult to detect tumors on a mammogram and are associated with an increased risk of developing breast cancer. Research in this area focuses on understanding the relationship between breast density and breast cancer risk, improving methods for measuring breast density, and developing personalized screening and prevention strategies for women with dense breasts.