Diffuse optics studies the propagation of light in highly scattering media (e.g. biological tissues). Time-resolved techniques – which measure the time-of-flight distribution of photons in the medium – permit to separate the contribution of absorption (linked to chemical composition) from scattering (produced by microstructure). Furthermore, for increasing photon time-of-flight, larger tissue depths are reached. These two properties, combined with the intrinsic non-invasiveness of the technique, are very interesting for clinical applications, such as diagnosis of breast cancer, imaging of brain activity, monitoring the functional and metabolic state of biological tissues in depth. The technology has undergone impressive growth, moving from laboratory laser systems to miniature wearable devices. To grant the reliability of the information obtained in vivo, international collaboration is needed to set objective validation protocols, in view of a growing diffusion of the method, from clinical diagnostics to home care.