In a continuum modeling approach of a multiphase flow, each transport phase is regarded as an individual pseudo-continuum fluid and all these “fluids” co-share the same space and time domains. Chapter 6 delineates the volume-averaging method to construct the pseudo-continuum fluids over which the volume-averaged Eulerian modeling approach is developed. The key concepts and formula of volume-averaged continuum modeling include the definitions of intrinsic and phase averages and their relationship; the volume-averaging theorems; the general form of volume-averaged transport equations and the individual equations of mass, momentum, and energy; the volume and mass balance conditions of all phases; the constitutive relations of the volume-averaged tensors by individual volume-averaged parameters; and the formulation of interfacial transport between phases. The effect of turbulence on phase transport is also handled via Reynolds decomposition and time-averaging over the volume-averaged equations. The detailed formulation of various turbulent transport coefficients and the salient behavior of the turbulence modulation from various interactions among phases are presented.