Photon migration in tissues has afforded a novel approach to the measurement of scattering (mus') absorption (mua) properties of human tissues. Localization depends upon the effects of localized absorbers upon the photon migration pattern. In this rapidly developing field, localization by the effect of localized absorbers/scatterers upon the mean pathlength observed between separated input-output points has afforded a primary approach. The pathlength is lengthened for a proximal object and shortened for an object distal with respect to the banana- shaped photon probability distribution. Such patterns are highly localized when the mean pathlength is sharply identified as by single frequency phase modulation systems. However, significant improvements are obtained when the optical pattern is more sharply defined. The possibility of localizing low concentrations of small volumes of blood and contrast agents is provided by the phased array optical system. The interference properties of the photon diffusion waves of wavelength at ~7 cm are described in experiments in collaboration with Gratton et al. (1). An array of laser diode transmitters with phases alternating between 0o and 180o in a four element array gives an antenna pattern containing a sharp null of 2 cm FWHM. When the array extends over one wavelength with transmitters at lambda/4, the sharpness of the null point is 5o of phase shift per min of displacement of the receiver with respect to the transmitter. the sensitivity in the detection of a 5-mm object containing a model contrast agent at the expected tissue concentrations is 1.5o/mm motion of the object midway between a 5-cm separation transmitter-receiver array in 0.5% intralipid, conditions closely mimicking those of breast or brain and at an agent concentration approximating the absorption of blood in tissue. No loss of sensitivity is obtained by measuring the time resolved fluorescence emission of a localized contrast agent. The estimated amount of cardiogreen required for 1.5o/mm sensitivity is 0.7 mug or 1.0 nmol. The localized perturbation of the phase array null by focused ultrasound will also be studied.