Since it is one of the most diffused forms of degradation of materials in ancient buildings, interest in biodeterioration is growing. One of the reasons for this may be that in addition to its esthetical impact, many experts see biodeterioration as the first step to even more severe forms of technical degradation.

In this contribution, first the most common forms of degradation of materials in monuments are described. Special attention is paid to biodeterioration and favourable conditions leading to it. The situation of monuments in Olinda (Brazil), which are suffering from defacement due to biological growth is described. Finally an approach to dealing with biological growth is given.