The Rare Breeding Birds Panel's twenty-first report provides a summary for the whole of the United Kingdom for 1993. Heavy rains at critical periods proved to be detrimental for a number of species. The year's main features included: continued declines of both Great Bitterns Botaurus stellaris and Corn Crakes Crex crex; but continued increases of Honey-buzzards Pernis apivorus, Red Kites Milvus milvus, Marsh Harriers Circus aeruginosus, Northern Goshawks Accipiter gentilis and Ospreys Pandion haliaetus; a record year for both Spotted Crakes Porzana porzana and Dartford Warblers Sylvia undata; and a welcome revival for Marsh Warblers Acrocephalus palustris. Black-winged Stilts Himantopus himantopus attempted to breed. Various other vagrant species showed signs of breeding behaviour, the most significant, perhaps, being those involving Little Egret Egretta garzetta, Great Reed Warbler Acrocephalus arundinaceus and Greenish Warbler Phylloscopus trochiloides.This is the twenty-first annual Report of the Rare Breeding Birds Panel. It includes the details of the breeding status of each species on the Panel's list which possibly, probably or certainly bred in 1993. The information presented varies according to the perceived sensitivity of the species to disturbance or to nest robbery. Thus, for the majority, the counties are grouped into the Panel's ten regions (see page 65), though for some it is possible to list the actual counties, whereas for others only country totals are given. This report is being published more than two years after the end of the calendar year with which it deals.