Historically there have been two major formulations for the mass-flux relations in multicomponent diffusion: (1) a generalization of Fick＇s law in which the mass flux is written as a linear combination of concentration gradients and (2) a generalization of Maxwell＇s expression in which the concentration gradient is given as a linear combination of the mass fluxes. The thermodynamics of irreversible processes has made it possible to generalize these expressions to include thermal, pressure, and forced diffusion. Associated with these two formulations, there are various definitions for the multicomponent diffusivities. Here the interrelations among the variously defined diffusivities are given, some connections with molecular theories are made, and several neglected publications are cited.