A bioreactor is a manufactured device or system that supports a biologically active environment. It can refer to a vessel in which a chemical process is carried out that involves organisms or biochemically active substances derived from such organisms, and this process can either be aerobic or anaerobic. Bioreactors are commonly cylindrical, ranging in size from liters to cubic meters, and are often made of stainless steel. Bioreactors can also refer to a device or system designed to grow cells or tissues in the context of cell culture, which is being developed for use in tissue engineering or biochemical/bioprocess engineering. Bioreactor design is a relatively complex engineering task, which is studied in the discipline of biochemical/bioprocess engineering. The environmental conditions inside the bioreactor, such as temperature, nutrient concentrations, pH, and dissolved gases (especially oxygen for aerobic fermentations) affect the growth and productivity of the organisms. Bioreactors are generally used in industries concerned with food, beverages, and pharmaceuticals.

Some key features of bioreactors include:

• Substrates: Bioreactors convert substrates to products, byproducts, and wastes, or eliminate water-soluble organics.
• Cultivation principles: Bioreactors can be characterized by the cultivation principle and the operation mode. The cultivation principles of microorganisms are further subdivided into submerged, immobilization, and solid state.
• Fermenter: The term fermenter is used when the application is the cultivation of bacteria, yeast, or fungi. In turn, the optimal H/D ratio of the vessel for microorganism cultivation is 3:1, but in the case of cell cultures, it is 2:1.

Bioreactors are used in industrial processes to produce pharmaceuticals, vaccines, or antibodies, and to convert raw materials into useful byproducts such as in the bioconversion of corn into ethanol.