The purpose of this paper is to summarize the effect of the excluded volume on interdiffusion phenomena in a ternary system.  The discussion on the concentration dependence of the four diffusion coefficients is based on the hydrodynamics of hard spheres.  Experimental data of several ternary systems containing macromolecules are presented for supporting the theoretical approach.  The most recent applications of mutual diffusion coefficients in thermodynamic analyses are resumed.  The future perspectives on this research are discussed.