Abstract: Density wave (continuity wave, voidage wave, kinematic wave) is a special phenomenon of two phase flow. The main developments of density wave theories were reviewed in this paper. These theories consider not only the characteristics of linear and non linear waves, but also the flow pattern transition, the formation of slug and the collapse of fluidized bed. The principal conclusions obtained from experiments and theories for propagation velocity, frequency, stability and dispersivity were discussed. The...