Irrespective of the method used, non-invasive measurement of arterial pressure may provide misleading information. The accuracy and performance requirements should be of the highest level possible for all instruments designed for pressure measurements in the perioperative environment. Invasive pressure measurement is prone to errors and must be considered a relative standard. The proper choice of arterial site, adequate handling of material and equipment, and prudent clinical interpretation are required to upgrade the invasive measurement to high-quality arterial pressure monitoring.