This article reviews the general features of the multiscale structures in particle-fluid systems and the characterization, modeling, and simulation methods for these systems. The discussion focuses on the effects of mesoscale behavior, especially those present in process industries for materials and energy transformation and utilization. When there is substantial multiscale heterogeneity in these systems, local non-equilibrium and anisotropy generally lead to a lack of scale separation. Accurate and efficient simulation methods based on first principles and applied across different scales are highly desirable to reveal and quantify the complexities of these systems. Meanwhile, precisely designed experiments and exhaustive nonintrusive measurements are necessary to validate and expand the numerical findings. With this knowledge, rational mesoscale models can be established to provide multiscale simulation methods that do not need to fully reproduce the micro- and mesoscale details of the systems but can still take into account their effects on macroscales. Such multiscale methods are attractive for industrial applications but substantial effort in physical modeling and numerical implementation is still required before their widespread implementation. (C) 2019 Elsevier Ltd. All rights reserved.