Ancient monuments in Greece are threatened by weathering. Soft limestones decay rapidly; marbles are more resistant. Various forces of natural weathering cause the decompositon of building stones, in concert with ground moisture and efflorescence of salts. According to the author air pollution has no influence on the decay of stone even in Athens. Biological attack may be very strong; lichen attack on marble can frequently be observed. Visitors cause deterioration if they are allowed to walk on ancient walls. Treatments are discussed.