This book puts the theory for a mixture of hypoplastic constituents on a rigorous basis using the thermodynamic analysis of Müller and Liu. The results which are specialised to frictional solid-fluid mixtures can be used by geophysicists and geotechnical engineers to refine their existing models for debris flows. This book addresses a continuum mechanical formulation of structured solid-fluid mixtures for elasto-viscous-frictional materials, suitable for particle laden and dense solid-fluid flows under small to catastrophic motions. Volume-fraction densities and balance laws for stress-like variables describe effects of the microstructure and hypoplasticity, and non-vanishing production rate densities of mass and momentum account for the description of compressibility or density preserving of the constituents, fragmentation, abrasion and particle separation. Saturation is equally incorporated. Using Müller’s entropy principle, paired with a number of plausible ad-hoc assumptions, the theory delivers explicit expressions for the constituent stresses, interaction forces, entropy, heat and entropy flux. A reduced model for a binary mixture yields an extension of the hypolasticity to multi-phase systems and points at the restrictions of the popular stress equilibrium assumption.