Packed bed reactors are commonly used for carrying out heterogeneous chemical reactions, and the attendant complexities arising out of the interactions of the momentum, heat, and mass transport processes make the design of such units a very involved and cumbersome task. The strategy for rational design depends strongly on the nature of the reaction scheme and its sensitivity to perturbations in operating conditions. Models ranging from simple one-dimensional pseudohomogeneous to two-dimensional heterogeneous are used to describe process behavior. Reviews detailing these aspects of the packed-bed have appeared from time to time.