Direct measurement of physiologic systems is often impractical. To overcome these obstacles, indirect physiologic measures have been developed. Indirect physiologic measures such as heart rate, blood pressure, and many others are surrogates that are believed to accurately represent the function of a physiologic system. Although a powerful tool, physiologic measurement has several potential limitations and errors. This can result in erroneous instrument data. For that reason, it is the responsibility of the clinician to question and interpret monitor output and to ultimately correctly assess validity of the measurement. This article reviews commonly used intraoperative monitoring techniques and discusses their potential limitations as they relate to hypovolemia and hemorrhagic shock.