A fermentor (fermenter) is a vessel for the growth of microorganisms which, while not permitting contamination, enables the provision of conditions necessary for the maximal production of the desired products. In the chemical industry, vessels in which reactions take place are called reactors. Fermentors are therefore also known as bioreactors. Fermentations may be liquid (submerged), or solid state (surface or solid substrate). Most fermentations used in the industry are submerged processes, because the fermentors used in submerged fermentation save space and are more amenable to engineering control and design. Depending on the purpose, fermentors can be as small as 1 liter or up to about 20 liters in laboratory-scale fermentors and range from 100,000 liters to 500,000 liters (approximately 25,000–125,000 gallons) for factory or production fermentors. Between these extremes, pilot fermentors are found. This chapter describes fermentors and operation of fermentation equipment, the aerated stirred-tank batch fermentor, construction materials for fermentors, aeration and agitation, temperature control, foam production and control, process control, anaerobic batch fermentors, fermentor configurations, continuous fermentations, fed-batch cultivation, air lift fermentors, microbial experimentation in the fermentation industry, the place of the pilot plant fermentor, inoculum preparation and surface or solid state (substrate) fermentors.