In the first part the fundamentals of controls which are applicable when either physical or chemical methods are used. In the second part the physical agents and physical methods for reducing or eliminating the viable microbial population. Microorganisms are not simple physical targets. Many biological characteristics influence the rate at which microorganisms are killed or inactivated by various agents. Many factors must be considered in the application of any physical or chemical agent used to inhibit or destroy microbial populations. It is not possible to prescribe one agent that will be effective for the control of microorganisms for all materials and all circumstances. The physical and chemical substance would carry the organism.