This paper reviews correlations involving the entrainment behavior of particles, such as catalysts, in fluidized bed reactors, in particular the dependence on particle size, density, concentration, gas velocity, density, etc. Some of the published correlations are criticized as obviously failing in certain situations. The author makes an analogy of a liquid boiling and ejecting droplets from the surface which either fall back or finally gradually evaporate and rise and builds his theory on this behavior. The paper includes a discussion of bubbling fluidized beds and circulation of solid particles in such beds. The paper reviews a considerable amount of experimental data. (LTN)