The importance of correct identification of the humidity source which may be damaging wall-paintings is emphasized. Four case-histories are then described: the 7-8th century church of S. Pietro in Valle, Ferentillo (Italy) had rising damp as a result of rainwater falling onto the outer walls plus condensation problems; improvement in drainage and installation of underfloor heating was the cure. The 8th-century church of Sagra, Carpi (Italy) had rising damp due to a surface groundwater table plus condensation; diagrams and photographs show how the walls were cut for insertion of a damp-proof course. The Museum of Kariye Cami in Istanbul (Turkey) had rising damp from poorly dispersed drainwater, plus hygroscopic humidity and condensation; improved drainage and internal slight heating plus a reduction in the number of visitors was the solution. The subterranean basilica at Porta Maggiore, Rome, is used as an example of an incorrect intervention (1950). Photographs, diagrams, plans.