Our portable clinical instrument for time-resolved optical mammography operates in transmittance geometry on the mildly compressed breast. Time-resolved transmittance data are collected at seven red and near-infrared wavelengths (i.e., 635, 680, 785, 905, 930, 975, and 1060 nm), using picosecond pulsed diode lasers as light sources, and two photomultiplier tubes and personal computer boards for time-correlated single photon counting to detect the time distributions of the transmitted pulses. Injection and collection fibers are scanned in tandem over the compressed breast and data are stored every millimeter. Images are routinely acquired from both breasts in cranio-caudal and oblique (45 deg) views. Time-resolved spectral data are interpreted with the solution of the diffusion equation for an infinite homogeneous slab, using a spectrally constrained global fitting procedure to estimate tissue composition in terms of oxy- and deoxyhemoglobin, water, lipid, and collagen content, as well as scattering parameters (amplitude and power ).15 Moreover, for the detection of breast lesions, scattering maps are routinely applied, together with late gated intensity images that are sensitive to spatial changes in the absorption properties. Details on the instrument setup and performances, and on the procedures for data acquisition and analysis, are reported in 16.