A direct consequence of the complex fluid mechanics of the circulating fluidized bed (CFB) is its complex mixing behavior. For both, solids and gas mixing, different scales of the mixing processes may be distinguished, namely the micro, meso and macro scales. While the micro scale is connected to the molecular scale and is mostly described by transfer coefficients, the meso and macro scales, are directly related to the flow pattern inside the CFB. On the meso-scale mixing effects are associated with the formation, disintegration and movement of clusters and strands. Macro-scale mixing is introduced via the existence of large-scale flow patterns, e.g. the core annulus structure. The present paper intends to give an overview on the mixing in a circulating fluidized bed and on its effects on the performance of CFB reactors. Special attention will be given to the interdependence between the reaction rate and the requirements for mixing. The combustion process in a CFB will mainly be used to exemplify this interdependence.