Suspension-firing, fluidized bed combustion and grate-firing are the three main technologies used to co-fire coal and biomass for heat and power generation. Computational Fluid Dynamics (CFD) modeling is a powerful and cost-effective tool for problem-shooting, design and optimization and plays a vital role in the development of these co-firing technologies. This chapter focuses on CFD of coal and biomass co-firing. After a brief overview of the status of co-firing, the chapter presents in detail the basic modules in general co-firing CFD. Then, the specific or new modeling strategies and issues that are pertinent to each of co-firing technologies are elaborated. Finally, new modeling issues for coal and biomass co-firing under oxy-fuel conditions are presented, before concluding remarks on coal and biomass co-firing modeling are summarized.