The possible functions of male advertising song in the Sedge Warbler (Acrocephalus schoenobaenus) and the Reed Warbler (A. scirpaccus) were studied using both observational and experimental techniques in the field. The seasonal rhythms of advertising song in the populations over three years were found to be species-specific, and were related to male arrival, female arrival and pairing. Population song was also related to population breeding acivity as revealed by the seasonal pattern of egg-laying. When population song was more synchronised, population breeding activiy was more synchronised towards the start of the breeding season. The diurnal rhythms of advertising song in individual marked males were monitored before and after pairing. In Sedge Warbler males advertising song ceased completely after pairing, and only resumed if the female deserted. In Reed Warbler males, a vestige of the former diurnal rhythm remained after pairing, with groups of paired males performing dawn and dusk choruses of 'social song'. The interspecific differences were extended further, as both observations and play-back experiments confirmed that paired male Sedge Warblers did not utilise advertising song in territorial interactions, but performed visual 'threat' displays instead. Male Reed Warblers however utilised advertising song in territorial interactions both before and after pairing. A functional interpretation of advertising song is presented, based upon the observational and experimental evidence collected. It is suggested that the prime function of persistent advertising song in the unpaired male is sexual attraction of the female, and that intermittent advertising song from paired males functions in territorial defence. The precise function(s) of social song in paired males are not known, but some suggestions are discussed. The interspecific differences in the functional spectra of advertising song found in the Reed Warbler (sexual, territorial, social) and Sedge Warbler (sexual only,) are discussed in relation to habitat and social organisation. The Reed Warbler inhabits more dense, non-visual habitats, and the Sedge Warbler more open, visual ones. Thus the Sedge Warbler has developed comparatively more visual forms of advertisement, and the Reed Warbler more vocal forms which in the latter case is reflected particularly in the wider functional spectrum of advertising song. The only common function for which advertising song is utilised in both species is sexual attraction of the female. This, and other evidence discussed suggests that sexual attraction may be the most important and perhaps the most primitive function of advertising song in these species.