Describes the problem of parasitical vegetation growing on stone monuments, how it develops, and possible counter-measures to take. Factors which encourage such infestation are: air-humidity; substrate humidity;temperature; rainwater; wind; and light; dissemination of spores (by wind, man, etc.) Vegetal infestation can affect the very stability of the monument (cracking, flaking). The paper outlines two basic methods of treatment: manual weeding; and chemical action - herbicides, chemical solutions, etc. After treatment, a surface protection such as vinyl acetate may be finally applied. -- ICCROM