The maintenance of a defined free H+ concentration within narrow limits is a prerequisite and feature of living organisms. In recent years the different disciplines of biological science have made considerable progress in the elucidation of the mechanisms involved in pH homeostasis. Recent advances have occurred also in the field of pH measurement. This review focuses mainly on the modern instruments for pH and blood gas analysis. The techniques of intracellular pH currently in general use are described together with some of the techniques for determination of pH in subcellular compartments.