This overview examines the heat and mass transfer characteristics in three-phase (gas-liquid-solid and liquid-liquid-solid) fluidized beds to provide prerequisite knowledge for reactor design. The effects of gas and liquid velocities, solid and liquid properties on the heat and mass transfer coefficients in three-phase fluidized beds have been determined. The various correlations and models to predict the heat and mass transfer coefficients in the literature have been examined and the unified correlations based on the concepts of surface renewal theory and energy dissipation rate in the beds have been proposed. The analogy between the heat and the mass transfer in three-phase fluidized beds have been discussed. The areas wherein future research should be undertaken to improve the state of the present knowledge are defined with recommendations. (C) 1997 Elsevier Science Ltd.