SUMMARYA multiwavelength, high bandwidth (1 GHz) frequency-domain photon migration (FDPM) instrument has been developed for quantitative, non-invasive measurements of tissue optical and physiological properties. The instrument produces 300 kHz to 1 GHz photon density waves (PDWs) in optically turbid media using a network analyser, an avalanche photodiode detector and four amplitude-modulated diode lasers (674 nm, 811 nm, 849 nm and 956 nm). The frequency-dependence of PDW phase and amplitude is measured and compared to analytically derived model functions in order to calculate absorption, µ a , and reduced scattering, µ s , parameters. The wavelength-dependence of absorption is used to determine tissue haemoglobin concentration (total, oxy-and deoxy-forms), oxygen saturation and water concentration. We present preliminary results of non-invasive FDPM measurements obtained from normal and tumour-containing human breast tissue. Our data clearly demonstrate that physiological changes caused by the presence of small (about 1 cm diameter) palpable lesions can be detected using a handheld FDPM probe.