AbstractBy combining near‐infrared intensity‐modulated illumination with frequency domain detection methods, we have obtained the first detailed optical images of thick tissues (1–3 cm). The raw data of these images are characterized by millimetre resolution and high contrast. The method employs a pulsed laser coupled to a cross‐correlation frequency domain detection scheme. The near‐infrared imaging method is non‐invasive and inexpensive, uses non‐ionizing radiation and is potentially fast enough for real‐time imaging in the seconds time range. In the frequency domain, diffusional wave optics provides the framework for a description of the light propagation in tissues. Our approach results in a theoretically and experimentally simple way to exploit the advantages of time resolution in optical imaging of inhomogeneous highly scattering materials. At any given modulation frequency, the light wavefront advances at constant velocity in a spherical wave. Objects with scattering or absorption coefficients that are different from the surrounding medium cause a deformation of the propagating wavefront that can be accurately measured using frequency domain detection methods. The visualization (on a computer screen) of these wavefront deformations provides a projection of the i