Granular matter is a large assemblage of solid particles,which is fundamentally different from any other type of matters,such as solid and liquid.Most models presented for granular matter are phenomenological and are only suitable for solving engineering problems.Many fundamental mechanical problems remain open.By analyzing characteristics of internal state structure,we propose that granularmatter is intrinsically multiscale,i.e.microscale of particle size,mesoscale of force chain,and macroscale of the bulk of granular matter.The correlations among difference scales would be crucial.The mesoscale force chain network is determined by both particle properties and macroscopic boundary conditions.The evolution of the force the chain network contributes to macroscopic mechanical properties of granular matter.In addition,we discuss the drawbacks in simplifying contact forces in the current models,and the difficulties in analyzing the interaction of interstitial fluid in wet granular matter.As an appropriate application of granular matter,debris flow can be studied with granular matter mechanics;meanwhile,debris flow brings more challenges which certainly motivate future studies on granular matter.(C) 2008 National Natural Science Foundation of China and Chinese Academy of Sciences.Published by Elsevier Limited and Science in China Press.All rights reserved.