The development, production, stockpiling and use in war of biological and toxin weapons (BTW) are prohibited by international law. However, conversion of BTW facilities has not been dealt with in the existing, open literature on conversion. This is partly due to the fact that BTW programmes to a large degree involve dual-use activities and it is thus difficult to differentiate them form defensive and civilian activities involving pathogenic biological agent and toxins. It is thus also difficult to assess whether or not a BTW facility has actually been converted. This text discuss general aspects of conversion and describe the conversion of former facilities in Germany, the UK, USA, Russia, and Kazakhstan.