The genus Peltigera (Lichenes) in Belgium and the Grand Duchy of Luxembourg. - The taxonomy, the distribution and the ecology of the genus Peltigera in Belgium and Grand Duchy of Luxembourg have been revised based on the study of 1272 herbarium specimens and on field work. An identification key for the 17 recognized species is provided. Of special interest are the results on the taxonomy of the P. canina group which contains eight species in the studied area, and of the P. polydactyla group which is represented by five species. Further, P. malacea and P. venosa, two species believed to have become extinct a long time ago in the studied area, are shown to have survived as small populations (until the seventies for the latter). The spectacular decline of the genus is mainly ascribed to air pollution, especially by nitrates, and to modern techniques of forest exploitation. The lichenicolous fungi Arthonia fuscopurpurea (Tul.) R. Sant. and Pronectria ornamentata (D. Hawksw.) Lowen are reported for the first time from the studied area.