Introduction Blood pressure (BP) is the one of the most commonly measured parameters of the cardiovascular system. The American Society of Anesthesiologists (ASA) guidelines for basic anesthesia monitoring, available online at http://www.asahq.org/publicationsAndServices/standards/02.pdf (last amended October 2005), mandate that arterial blood pressure be determined and evaluated at least every five minutes. Numerous methods of noninvasive BP measurement, next to the invasive assessment via an indwelling catheter, are clinically available. Technical concepts, evidence of utility, indications, contraindications, and complications of blood pressure monitoring are discussed in this chapter. Technical concept General principles Arterial BP, typically recorded as a systolic, diastolic, and mean arterial pressure (MAP) in millimeters of mercury, is derived from the complex interaction of the heart and the vascular system and varies not only at different sites in the body, but also when individual components of this system are changed. Stroke volume, systemic vascular resistance, the velocity of left ventricular ejection, the distensibility of the aorta and arterial walls, and the viscosity of blood all are components of this complex interaction that ultimately results in a pressure reading.