<p>In large-scale bioprocesses, gradients in pH, dissolved oxygen level (DO), and substrate concentrations can decrease bioprocess efficiency. Scale-down bioreactors, be it single stirred-tank bioreactors with a special feeding regime, multi-compartment bioreactors, or combinations of bioreactors, offer a promising lab-scale solution for comprehending these gradients, as they allow adjustment of gradients without incurring high costs. However, critical challenges arise when transitioning from large-scale to scale-down bioreactors. Chief among these is realistically approaching the gradient conditions of large-scale bioreactors and choosing appropriate scale-down bioreactor configurations. This review paper begins by addressing the gradients encountered in large-scale bioreactors. Afterward, various types of scale-down bioreactors are characterized and compared, highlighting their advantages and disadvantages. The suitability of scale-down bioreactors is analyzed by examples of bioprocesses with different microorganisms and mammalian cells to underscore the complexities inherent in scale-down bioprocesses and emphasize the influence of cellular responses to these conditions. Finally, the potential of miniaturized and microfluidic bioreactors is briefly discussed for future application in scale-down studies. </p>