This chapter starts from the observation that solid tumours can be considered as porous materials existing in a liquid environment permeating the interstitial space and in presence of other structure such as the extracellular matrix network. It is shown how in this framework it is possible to deduce Darcy’s law to describe the flow of the extracellular fluid in the multicellular aggregate and the advection-diffusion of proteins and drugs. Finally, the modelling framework is applied to describe solid tumour growth in vitro as a biphasic porous material with nutrients.