Recent developments in the field of photoelectron spectroscopy (pes) of transient species are reviewed. An outline is given of the basic apparatus necessary for such studies and theoretical methods used to interpret the spectra are discussed. Examples are chosen of short-lived atoms and small molecules which have been studied by pes to illustrate the type of information to be obtained with this technique. Progress in studies of large molecules, van der Waals complexes, radicals adsorbed on surfaces and high temperature species, is also discussed. Suggestions are made regarding likely future developments in these areas.