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All microorganisms require water, sources of energy, carbon, nitrogen, mineral elements, and possibly vitamins plus oxygen if aerobic. On a small scale it is relatively simple to devise a medium containing pure compounds, but the result- ing medium, although supporting satisfactory growth, may be unsuitable for use in a large scale process. On a large scale one must normally use sources of nutrients to create a medium which will meet as many as possible of the following criteria: 1. It will produce the maximum yield of product or biomass per gram of substrate used. 2. It will produce the maximum concentration of product or biomass. 3. It will permit the maximum rate of product formation. 4. There will be the minimum yield of undesired products. 5. It will be of a consistent quality and be readily available throughout the year. 6. It will cause minimal problems during media making and sterilization. 7. It will cause minimal problems in other aspects of the production process particularly aeration and agitation, extraction, purification, and waste treatment.