pH is the way of expressing the hydrogen ion concentration in water. It is related to the acidic or alkaline nature of water. Consideration of hydrogen ion concentration is important in almost all uses of water including efficient operation of the wastewater treatment system. pH balance is important in maintaining the desirable equator ecological conditions in natural water. The pH of most natural water lies between 6.5 and 8. This chapter explains the pH scale, method of calculation of PH, pH indicator and the use of pH metre.