The trilogy on transport phenomena ends with the chapter dedicated to the mass transport. We will begin with molecular transfer, that is, diffusion, presenting Fick’s law and the diffusion coefficient, with the associated Schmidt number that represents the ratio between the molecular transport of momentum and mass. We will then address the problem of transfer in the presence of convective motions, appropriately defining the transport coefficients and the Sherwood number, seen as the ratio between the total transport and the molecular transport of mass. In particular, our attention will focus on the transfer of mass between phases, in the hypotheses of validity of the film theory. We will identify the expressions for flux, resistances, local and global transfer coefficients.