Arterial blood pressure (BP) measurements, which include invasive direct methods and noninvasive indirect methods, provide a picture of the hemodynamic status of the patient. Invasive BP methods measure pressure pulse wave amplitude; noninvasive methods rely on blood flow or arterial wall motion as a basis for the determination of BP values. To obtain the most accurate BP value, the clinician must identify which measurement variables in a specific clinical situation are most contributory to error and, if possible, use a method of measurement for which the sources of error are not parallel. Blood pressure values obtained by different methods cannot be compared without a thorough understanding of the user-related and instrumentation-related limitations associated with each BP measurement technique