This Working Document is written for government and industry risk assessors, and for scientists involved in the registration and regulation of microbial pest control products (MPCPs) and their active agents (MPCAs). However, it can also be a useful tool in the assessment of microbial biocides. It presents the views of the different OECD countries on how they address these scientific issues in the safety evaluation of MPCPs. It is intended to be used as guidance in the safety assessment of microbials, but its use is not a requirement. For example, it can be used for the 4th list of substances to be assessed by the EU, for re-registrations of microbials, for national authorisations. In this way, government safety evaluations of data submitted for registration can be improved. With this guidance, (i) companies should be better prepared to submit the relevant data for risk assessment and (ii) regulatory authorities should be better prepared to review the submitted dossiers and monographs.