Until now, cleaning monuments has been discussed much less than, for instance, cleaning paintings. Nevertheless, washing and chemical cleaning is widely used despite several drawbacks and failings. Even cleaning with pure water is not to be recommended, because it normally results in wet walls and basements, moving internal salts, changing the interior climate, etc. Under high pressure, water as any other equipment, if not used very carefully by conservators, is no less dangerous to weakened stone, plaster, stucco or polychromy and may destroy delicate details. Following the use of acid or lyes, destructive deposits are usually left within the porous material as recent analyses of stone sculptures and buildings in Austria have shown. To illustrate the actual dangers involved in cleaning, recent examples of primary and secondary failures of cleaning are given, including the activation of salts on stone sculptures inside a museum and mold growth on church paintings. More research and a better exchange of experience is required for the future. -- AATA