There is still a lack of commercial applications of pH sensors due to the decisive disadvantage that the signal is depending on the ionic strength of the sample. pH is defined as the negative logarithm of the activity of protons in aqueous solution, while the optical parameter (e.g. change of absorption or fluorescence) correlates with the concentration of the pH-sensitive dye. The cross-sensitivity of the calibration curve towards ionic strength can cause pH errors of up to 1.5 pH units. Since the electrolyte concentration, respectively the ionic strength, is not constant in most real samples, optical pH sensors have not often been applied in these systems. In systems with constant ionic strength (e.g. blood, sea water) pH was successfully determined via optical sensors. Therefore, there is a need for optical sensors displaying a comparably negligible effect of ionic strength on the measured signal. This work describes the fabrication and application of fluorescent pH sensors with special attention to minimize of the effect of ionic strength on the sensor. The book is written for students, scientists and people interested in the problems of measuring pH with optical sensors.