Publisher Summary This chapter focuses on purine nucleoside phosphorylases (PNPases), which are widely distributed in nature and have been identified and studied in a number of tissues in mammals, chickens, fish, yeast, and in several species of bacteria. In the erythrocytes of dogs, the activity of PNPase is remarkably low when compared with the red cells of other species. In a study of PNPase in developing chicken liver, it was found that the activity remained constant throughout the embryonic state however increased rapidly to about double the original activity in the first four days after hatching. In some bacteria, PNPase is subject to induction. Purine nucleoside phosphorylases catalyze readily reversible reactions, and there has been speculation that they are involved in both the synthesis and breakdown of nucleosides. As there are nucleoside kinases specific for several of the pyrimidine nucleosides, such as thymidine, uridine, and deoxycytidine, it is clear that pyrimidine nucleoside phosphorylases can function in the pathway of pyrimidine nucleotide synthesis. A significant body of evidence indicates that PNPase can participate in nucleoside exchanges, often in coupled reactions with a pyrimidine nucleoside phosphorylase.