Publisher Summary This chapter presents a discussion on application of hydrostatic pressure to microbial cultures. Hydrostatic pressure is generally not employed by investigators working with organisms that normally live at 1 atm. However, in the aquatic environment the Ideal Gas Law must be taken into consideration, because all organisms living below the surface of the water live under various degrees of hydrostatic pressure. Hydrostatic pressure as it affects micro-organisms is reviewed by several researchers. Hydrostatic pressure equipment designed by various investigators is listed by Morita, who includes such items as pressure viscometers, optical rotation cells, and electrochemical measurements under pressures. Because micro-organisms as well as the various sub-units of micro-organisms are bathed in an aqueous menstruum, the investigator should always take into consideration the effect of hydrostatic pressure on the physical and chemical environment. Some of the factors that the investigator should be cognizant of are the pressure effects on ionization of water, viscosity of fluids, pH, chemical reaction rates, ionization of various substances, hydrophobic bonding of proteins, and viscosity of water. Various types of pressure pumps have been employed to generate the needed hydrostatic pressure. The most widely used type in the United States for biological research is a modification of a high-pressure hydraulic truck jack in which a substitution of a mixture of glycerine and water (1: 1) for the hydraulic fluid is employed.