A debris flow is a flow of sediment-fluid mixture. Four key features in momentum exchange of debris flows are fluid viscosity, turbulence, particle sliding friction, and particle collision. Debris flows were qualitatively classified into six flow regimes, according to the dominance of these key features. Existing rheological models for debris flows in various flow regimes were briefly reviewed. The characteristics of flow velocities for a steady, two-dimensional uniform debris flow in each flow regime were obtained by treating the debris flow as a single-layer uniform mixture. The mixed-layer models and the hydraulics of debris flow were also discussed.