The term control as used here refers to the reduction in numbers and/or activity of the total microbial flora. The principal reasons for controlling microorganisms are to prevent transmission of disease and infection, to prevent contamination by or growth of undesirable microorganisms and to prevent deterioration and spoilage of materials by microorganisms. Microorganisms can be removed, inhibited or killed by various physical agents, physical process or chemical agents. A variety of techniques and agents are available. They act in many different ways and each has its own limit of application.