This chapter introduces a multiscale method for modeling gas–solid flows, from steady-state modeling for quick engineering design, continuum model for industrial scale reactor simulation, discrete particle method for lab-scale reactor simulation to direct numerical simulation for exploring the underlying mechanics. Numerous experimental or empirical correlations for fluidized bed systems have been put forward. Fluidization is a process where solid granules are transformed into a fluid-like state through suspension in a stream of gas, liquid, or their mixture. Local heterogeneity in gas–solid fluidized beds is characterized by a two-phase structure consisting of a particle-rich dense phase and a gas-rich dilute phase. A typical fluidization system such as CFB riser contains both local mesoscale heterogeneous structures and overall nonuniform distribution of voidage or solids concentration in the axial and radial directions. Steady-state modeling is usually the first option to realize real-time simulation of industrial processes for quick engineering design.