To form giant planets during protoplanetary disk lifetime, small micron sized particles must grow rapidly to larger grains. A full understanding of that process requires a detailed characterization of the radial and vertical structure of the gas-rich disks associated with young pre-main sequence stars. This is because the level of dust concentration controls grain growth efficiency and planetesimal formation. Multi- wavelengths observations of protoplanetary disks, for example in the millimeter and near-infrared, allow to probe very different grain sizes that are differently affected by evolutionary mechanisms. Here, I will discuss observational constraints on dust accumulation in early and evolved disks, with a particular focus on recent constraints on vertical dust concentration using multi-wavelength observations from various instruments, such as ALMA, JWST, HST… The modeling of multi-wavelength observations of inclined disks allows to identify high density regions, favorable for grain growth and planet formation, and to study the efficiency of planet formation in protoplanetary disks