Based on the substrate or media, the fermentation process is classified as submerged fermentation (SmF) and solid-state fermentation (SSF). SmF involves the growth of microorganisms as a suspension in a liquid medium in which various nutrients are either dissolved or suspended as particulate solids in the media. This process is carried out under aseptic conditions in a bioreactor or fermenter. SmF is well established on the industrial scale due to well-designed bioreactors and is highly suitable for microbial, plant, and mammalian cell cultures as they require high water content for their growth. SSF is defined as the growth of microorganisms on an insoluble solid matrix with minimal water content. Thus it is close to the microorganism’s natural habitat and is especially used for fungi. Bioreactor design is very crucial in transferring the process technology from shake flask to pilot or industrial scale. Bioreactors are classified into submerged reactors and solid-state reactors based on the physical state of the substrate or media used for the fermentation. The main criteria for the selection of bioreactors include culture conditions required for the selected microorganisms, simplicity in operation, scalability, material demand, and the cost of the equipment. Bioreactors can also be operated in batch, fed-batch, or continuous mode based on the nature of microorganism, substrate, and product. In this chapter, different types of reactors used for SmF and SSF will be discussed. In addition, important design parameters in terms of transport processes such as mixing, power requirement, and oxygen dispersion will be discussed, which play a major role in productivity of the reactors. © 2022 Elsevier Inc. All rights reserved.