In this chapter, scale‐down methodology is described as a useful tool to study the effects of fluctuating conditions on large‐scale bioreactors. The scale‐down approach focuses on simulating, on the laboratory scale, the environmental conditions prevalent on the large scale and analyzing the effects of these fluctuations on cell physiology. The scale‐down approach consists of the following steps: regime analysis of the large‐scale conditions, their simulation on laboratory systems, optimization and modeling of the cultures, and the application of the results on the large scale. These steps are presented and discussed. A review of the published scale‐down studies in cell culture and microbial fermentation is also included. Finally, an overview of the most commonly used scale‐down systems for simulating environmental fluctuations is presented, emphasizing their advantages and limitations for their use on microbial or animal cells models.