Suggestions by Calvin about a role of inorganic pyrophosphate (PPi) in early photosynthesis and by Lipmann that PPi may have been the original energy-rich phosphate donor in biological energy conversion, were followed in the mid-1960s by experimental results with isolated chromatophore membranes from the purple photosynthetic bacterium Rhodospirillum rubrum. PPi was shown to be hydrolysed in an uncoupler stimulated reaction by a membrane-bound inorganic pyrophosphatase (PPase), to be formed at the expense of light energy in photophosphorylation and to be utilized as an energy donor for various energy-requiring reactions, as a first known alternative to ATP. This direct link between PPi and photosynthesis led to increasing attention concerning the role of PPi in both early and present biological energy transfer. In the 1970s, the PPase was shown to be a proton pump and to be present also in higher plants. In the 1990s, sequences of H(+)-PPase genes were obtained from plants, protists, bacteria and archaea and two classes of H(+)-PPases differing in K(+) sensitivity were established. Over 200 H(+)-PPase sequences have now been determined. Recent biochemical and biophysical results have led to new progress and questions regarding the H(+)-PPase family, as well as the families of soluble PPases and the inorganic polyphosphatases, which hydrolyse inorganic linear high-molecular-weight polyphosphates (HMW-polyP). Here we will focus attention on the H(+)-PPases, their evolution and putative active site motifs, response to monovalent cations, genetic regulation and some very recent results, based on new methods for obtaining large quantities of purified protein, about their tertiary and quaternary structures.