The monitoring of the pH of the interstitial tissue fluid (tpH) has been reported to be of prognostic and diagnostic value in the surgical patient, in open heart surgery, and in pediatric surgery. Recent technical advances in pH probe and electronic design have made it practical to monitor the tissue pH in the fetus, newborn and adult. TpH measurements reflect the immediate cellular environment, and they are sensitive to decreases in local perfusion. Therefore, tpH may be different from Arterial pH (ApH) if there is a local disturbance of metabolism or flow. The use of tpH to monitor continuously the fetus during labor has been shown to be accurate and reliable, but this application is presently limited to research centers. Neonatal and adult tpH monitoring provides not only ApH, but the ApH-tpH gap provides clinically useful prognostic and therapeutic information. In neonates, tpH provides the earliest and most reliable measure of shock. The tpH is a practical and safe method for continuously monitoring pH, and it provides new and useful information for the care of the critically ill patient.