The behaviour of three-phase fixed bed reactors is complex due to hydrodynamic characteristics arising from the gas and liquid flow through the catalytic packed bed. This situation is particularly relevant for hydrotreatment processes studies in laboratory, bench-scale and pilot plant reactors. The effective use of these reactors requires either the evaluation of the influence of non-idealities on reactor conversions or bed modifications to minimize divergencies with respect to ideal reactor behaviour, namely ideal plug flow behaviour and complete external wetting of catalyst pellets. To achieve these requirements, three complementary tools have been analyzed: use of mathematical models to describe the behaviour of three-phase fixed bed reactors, use of simple criteria in order to verify whether hydrodynamic aspects affect the behaviour of experimental reactors and changes of either geometrical characteristics or operating conditions in order to achieve an ideal reactor behaviour. For the effective use of any of the proposed tools, reliable correlations of hydrodynamic and transport parameters are needed; therefore, a review of available correlations for the evaluation of axial dispersion, holdup, wetting efficiency and liquid-solid mass transfer is presented.