Electrospray makes use of ions present in electrically charged droplets in an aerosol. The generation of an aerosol by electrospray has already been published by Zeleny in 1917. The feasibility of electrospray as an ionization technique was demonstrated by Fenn and coworkers, and by a group of Russian scientists in Leningrad (St. Petersburg) in 1984. Interest from the mass spectrometric community and the instrument manufacturers rose dramatically in 1988, when Fenn and coworkers showed series of multiply charged ions produced by electrospray of protein solutions.The essential steps of electrospray: dispersion of a sample solution into a charged aerosol and the liberation of ions from droplets take place at atmospheric pressure. The sampling of ions from the atmospheric pressure region into the vacuum of the mass analyzer is a technical problem that has been solved in different ways by researchers and manufacturers. The principle of ion formation by electrospray seems simple but the mechanism is not known in detail. The majority of applications is in drug metabolism, peptide and protein research. The use of electrospray will spread to other fields of research that may benefit from the direct observation of ions in solution by mass spectrometry.