Time-resolved and spectroscopic in vivo measurements were performed to determine the optical properties of the female breast in transmission. The time-resolved measurements were carried out at different positions on the female breast with a Ti:sapphire laser (800 nm) using a synchroscan streak camera. A diffusion model was used to calculate the absorption coefficient A and the reduced scattering coefficient. In addition, spectroscopic in vivo measurements of more than 100 patients were performed in a wavelength range between 650 and 1000 nm. A variety of pathological alterations could be characterized by measuring patients of different ages, different breast sizes, and at varying locations on the breast. The results indicate that besides the pure detection of the amount of blood in the neovascular network, the volume concentrations of water and fat seem to be of particular importance for discrimination. In order to quantify this observation, an analytical model was developed that takes the volume percentages of fat and water, the concentration and oxygenation of hemoglobin, and the relevant optical parameters into account. Experiments were carried out with volunteers and patients in a clinical environment: Typical observations are presented and analyzed statistically.