Japanese concepts of modeling debris flow are thoroughly reviewed in this chapter. Many Japanese models, ranging from highly theoretical non-Newtonian fluid models to the very simple empirical relations of Bingham and Bagnold, are evaluated in terms of accuracy, generality, and practical usefulness, and are compared with a generalized viscoplastic fluid model described herein. Most debris flow formulas and criteria presently used in Japan are closely related to those developed by Takahashi on the basis of Bagnold’s “dispersive” pressure concept. Although the generality of Bagnold’s model is still at issue, Japanese scientists apparently have accepted Takahashi’s debris flow formulas and criteria. For example, Takahashi’s velocity profile for (steady) uniform debris flow in wide channels is only valid for grain-intertia regime. Applying Takahaski’s solution to modeling other flow regimes than the grain-inertia may thus have to adjust the value of Bagnold’s numerical constant in order to better fit the computed velocities to the measured ones. This and many other aspects of debris-flow modeling concepts in Japan are critically examined. An appraisal of the present status of Japanese research in debris flow modeling helps determine the direction of future efforts in debris flow research.