Proton exchange membrane fuel cells are at present in the forefront of all types of fuel cells because of the 'quantum jumps' in, its technology since its application in the 1960s, in NASAs Gemini space flights as an auxiliary power source. PEMFC technology since the. 1960s highlights the 'quantum jumps' both, in the fundamental areas of research (Part I), and in the engineering and technology and developmental (Part II), which are leading to the era of promising applications. This part of the review is on the 'quantum jumps' in the fundamental areas of research, which have enabled the rapid advances in the technology development, i.e. (i) transition from polystyrene sulfonic acid to perfluorosulfonic acid membranes; (ii) a 10-to 100-fold reduction in the platinum loading in electrode by using nanosize electrocatalyst particles supported on high surface area carbon and impregnation of the proton conducting electrolyte into the active layer of the electrode; (iii) optimization of structure of electrode and of membrane and electrode assembly to enhance power densities to 0.5-0.7 W cm(-2) at desirable efficiencies; (iv) using perfluorosulfonic acid membranes for DMFCs instead of liquid electrolytes to attain reasonably good efficiencies and power densities. The vital scientific challenges needing resolution for further advances in the technology are also summarized.