Even the best present day ultrahigh resolution electron microscopes embody a paradox. They are equipped with the most modern ion pumps, cryopumps or turbomolecular pumps, but the forepumps are oil-sealed rotary pumps invented by Gaede in 1905. Consequently efficient protective systems are necessary to prevent oil-derived vapour from contaminating the specimen, the objective lens aperture diaphragm, the field emission or lanthanum boride electron sources and other sensitive areas. Continuously operable, reciprocating, dry fore-vacuum pumps, based on our designs, have been made and sold by Varian for over three years and have proved reliable in a wide range of equipment. Electron microscopes too, should be provided with oil-free forepumps.The need for oil-free forepumps occurred to one of us in 1949 when it was shown that pump oil was the major source of the carbonaceous contamination of microscope specimens and objective lens diaphragms. Materials available at that time seemed unsuitable for the construction of oil-free forepumps but during 1968 we learnt that highly wear resistant, low-friction composites of polytetrafluoroethylene (PTFE) filled with finely divided bronze, graphite, etc. were being marketed. In 1970 we began to devise a practical oil-free pump. Rotary designs were rejected because all practicable shaft seals require oil lubrication to transfer heat from the seal to the shaft to prevent the seal material from reaching temperatures at which it would soften and wear rapidly. Each lubricated shaft seal would expose, at least, a narrow annulus of oil to the vacuum in the working chamber and seal wear would lead to substantial leakage of oil.