Pressure measurement has remained an important and feasible adjunct to clinical practice and scientists have sought better ways to measure it from desired points in the human body. Successful pressure monitoring has acquired an established role as it permits the accurate evaluation of the stage of the disease in heart, lung or other organ. This chapter is devoted to pressure in liquids, gases, its standard units and its measurement along with catheter transducer system. Due to inertia of mercury, classical mercury manometers are inadequate in recording pressure changes in critically ill patients. Electronic pressure transducers have been employed for recording rapid pressure changes. We hear of breakthroughs in the development of catheters transducer systems for measuring fast-changing pressure. This technique has demonstrated its efficacy in evaluation of patients in shock or patients with acute myocardial infarction. Further this chapter covers pressure profile in vascular tree and pressure volume relation in hollow organs. Hydrostatic pressure occurs in vascular system of human beings because of the weight of the blood in vessels. Due to this living system operates under continuous pressure but does not collapse. Effect of gravity on arterial and venous pressure and distribution of blood flow in lungs has been explained through diagrams and effect of pressure changes on human body during deep sea diving has been discussed.