A very brief survey about the Fick’s and thermodynamic diffusion coefficients and the main expressions of its prediction in gas, liquid, and electrolyte solution as well as dense and porous membranes is given. For dense membranes, the Flory–Huggins approach is recommended to predict the chemical potential and from that, the diffusion coefficient. For porous membranes, the diffusion coefficient is given, depending on the ratio of the pore size and size of the solute molecules. Mass transfer rate is defined due to the diffusion in concentrated solution. At the end, the ionic transport and the hindrance factors are listed depending on the molecule size and pore size.