Idealized mathematical models for multitubular packed-bed reactors, based upon simplifying assumptions concerning symmetry, regularity, and homogeneity of the packing and the intertubular space, are commonly used for analysis and design. In reality, substantial deviations from these assumptions occur and give rise to maldistributions, both for the flow of reactants through a packed tube and for the now of a heat-transfer medium through the intertubular space. These maldistributions and methods of how to deal with them in reactor design and simulation are reviewed in this paper. The following areas are covered : (i) porosity and now patterns inside tubes, (ii) heat and mass transfer inside tubes, (iii) flow distribution over individual tubes, and (iv) distribution of coolant flow over the intertubular space.