A time-resolved reflectance and transmittance spectrophotometer was developed and used to evaluate noninvasively and simultaneously the absorption and transport scattering spectra of bulk turbid media. The experimental setup, based on mode-locked dye and titanium:sapphire lasers and on an electronic chain for time-correlated single photon counting, was fully automated to make in vivo measurements on biological tissues feasible. The optical characterization of breast, of interest for the development of optical mammography, was performed in vivo on a wide spectral range (610–1010 nm). In particular, the absorption spectral features of the main tissue components (hemoglobin, water, and lipids) were recognized with different relative weight, depending on the age and the measurement position. Best fit with the absorption spectra of the main constituents was performed to evaluate their relative concentration in tissues. © 1999 American Institute of Physics.