Near-infrared spectroscopy (NIRS), which was originally designed for clinical monitoring of tissue oxygenation, has been developing into a useful tool for neuroimaging studies, whereas the central issue of NIRS, difficulty in selective and quantitative measurement of cerebral hemoglobin (Hb), remains to be solved. In continuous wave measurements, multi-distance probe arrangement and mathematical approaches, such as principal component analysis (PCA) and independent component analysis (ICA), are effective for selective detection of cerebral Hb changes, but are insufficient for quantitative measurements. By using time-resolved spectroscopy (TRS), we have been developing some approaches to these issues: (1) the method with analytical solutions of the diffusion equation, (2) the method with spatially resolved and time-resolved reflectances, (3) time-segmented analysis of time-resolved reflectances and (4) diffuse optical tomography (DOT). Among them, DOT, which allows 3-D quantitative imaging of optical properties, is thought to be the most promising for solving the central issue. However, the image quality of DOT is still low, because the inverse problems are usually nonlinear, ill-posed, and underdetermined due to the diffusive nature of the photon migration. In this symposium, I first summarize our approaches, and then describe the current status and future prospects of DOT.