Recent high resolution and high fidelity observations of protoplanetary discs are showing us peculiar substructures, such as spirals, gaps, rings and horseshoes. Disc hydrodynamical instabilities and embedded proto-planets have been proposed to be responsible for their formation. However, their origin is still very debated, in particular it is not clear how their morphological features depend on the disc mass and on the coupling between the gas and the dust component of the disc.
I will present an overview about dust and gas dynamics in protoplanetary discs, focusing on how much coupled they are. Then, I will present some results we have obtained in order to find if it is possible to estimate the gas disc mass in protoplanetary discs by the combined hydrodynamical modeling of planet-induced disc substructures observed in scattered light (micron-sized grains) and in the dust continuum (mm-sized grains).