This paper is devoted to the understanding of the structure heterogeneity, regime multiplicity and behavior nonlinearity of particle-fluid systems which give rise to predominant difficulties in their modeling and scale-up. Possible approaches are explored for dealing with these aspects of complexities. Two types of nonlinearity are recognized in particle-fluid systems - intrinsic, as related directly to particle-fluid interaction, and secondary, as related to particle geometric and system scale-up. It is indicated that multiple resolution with respect to energy, process, movement and structure may be a promising approach to coping with intrinsic nonlinearity, though understanding of secondary nonlinearity calls for more specific analysis of the effects of external factors on intrinsic nonlinearity.