Inorganic polyphosphate (poly P) is present in all species tested to date, from each of the three kingdoms of life. Studied mainly in prokaryotes, poly P and its associated enzymes are important in diverse basic metabolism, in at least some structural functions and, notably, in stress responses. These numerous and unrelated roles for poly P are probably the consequence of its presence in life-forms from early in evolution. The genomes of many bacterial species, including pathogens, encode a homologue of a major poly P synthetic enzyme, poly P kinase 1 (PPK1). Loss of PPK1 results in reduced poly P levels, and deletion of the ppk1 gene in pathogens also results in a loss of virulence towards protozoa and animals. Thus far, no PPK1 homologue has been identified in higher-order eukaryotes and, therefore, PPK1 exhibits potential as a novel target for chemotherapy.