Scale-up and commercialization of any chemical process is an inexact science and the scale-up of fluidized bed processes is especially so. The problem of gas bypassing and poor contact with bed particles has been recognized as a major scaling consideration for a generation. More recently, especially in circulating fluidized beds, the issues of particle flow patterns and density profiles have become appreciated as important unknowns in scale-up. Equally important scale-up difficulties occur outside the realm of reaction engineering, having to do with the physical nature of solid particles. The importance of all these factors will be discussed with reference to the commercialization of several fluid bed processes.