This chapter discusses the concentration-dependent interdiffusion coefficients in ternary alloy. The interdiffusion data at composition points of intersection of diffusion paths is provided. The chapter illustrates the schematic diffusion paths for a pair of solid-solid diffusion couples and a pair of vapor-solid diffusion couples. Interdiffusion coefficients at maxima and minima in concentration profiles are given. During isothermal diffusion in a ternary or multicomponent diffusion couple the interdiffusion flux of a component can go to zero at a section within the diffusion zone and exhibit a change of flux direction from one side of the section to the other. The ratio of the cross to the main interdiffusion coefficients can be directly determined from the slope of the ternary diffusion path at a zero-flux plane (ZFP) composition. A ZFP developed for a component in an experimental ternary diffusion couple is shown. The ZFP phenomenon has been identified in several multicomponent systems.