Main factors limiting downscaling of fixed-bed integral reactors are discussed, which operate with a gas stream or with gas and liquid in trickle flow, as used in hydroprocessing of oil. Criteria developed for a sufficiently close approach to plug flow and for good contacting of the catalyst show that they can be met in very small reactors, both for processes with reactants in the gas phase and for trickle-flow processes, provided that the catalyst bed is diluted with fine inert material in the latter case. Experimental tests show that microreactors with typical catalyst volumes of 5–10 mL can be used to obtain representative results that can very well match data from industrial reactors. It appears to be feasible to miniaturize catalytic test reactors further to “nanoscale” reactors with as little as 0.2–0.4 mL of catalyst while maintaining the results to be meaningful. Even though there are advantages as well as limitations, miniaturization can enhance safety and reduce manpower.