A 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, μa, 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.