Mammographic density refers to radiologically dense breast tissue, and reflects variations in the tissue composition of the breast. It is positively associated with collagen and epithelial and non-epithelial cells, and negatively associated with fat. There is extensive evidence that mammographic density is a risk factor for breast cancer, independent of other risk factors, and is associated with large relative and attributable risks for the disease. The epidemiology of mammographic density, notably the inverse association with age, is consistent with it being a marker of susceptibility to breast cancer. Cumulative exposure to mammographic density may be an important determinant of the age-specific incidence of breast cancer in the population. All risk factors for breast cancer must ultimately exert their influence by an effect on the breast, and these findings suggest that, for at least some risk factors, this influence includes an effect on the number of cells and the quantity of collagen in the breast, which is reflected in differences in mammographic density. Many of the genetic and environmental factors that influence the risk of breast cancer affect the proliferative activity and quantity of stromal and epithelial tissue in the breast, and these effects are reflected in differences in mammographic density among women of the same age. Some of these influences include endogenous and exogenous hormones, and the menopause. A better understanding of the factors that influence the response of breast tissue to these hormonal exposures may lead to an improved understanding of the aetiology of mammographic density and of breast cancer.