Many fuel conversion technologies using dual fluidized bed (DFB) have been developed, including the chemical looping combustion (CLC), CO2 acceptor, and DFB pyrolysis or gasification of biomass and coal. In this kind of system, solid particles are circulated between two fluidized bed (FB) reactors to carry heat or a specified chemical element from one reactor to the other. Thus, a special matching in heat and mass as well as its control is critical to the involved reactors and their integrated system. This paper is devoted to reviewing the recent progress on chemical engineering fundamentals particularly about the DFB fuel pyrolysis and gasification systems in terms of technical process, reactor and auxiliary equipments development, and bed material and reaction control. While the first aspect covers the optimization of reactor combination, process simulation, and hydrodynamic and kinetic modeling, the second part includes the optimization of reactor structure, realization of high particle circulation rate, and integration of reactor with siphon. The highlights in the third aspect are about the pretreatment of fuel and the choice of bed material in the view of intensifying reactions and upgrading products. It is expected that this Review not only summarizes the related studies but gains also an understanding of the DFB pyrolysis and gasification systems from an alternated viewpoint.