The main purpose of this Guide is to provide senior managers and operational staff of drinking water utilities a concise reference document on the requirements for optimising the processes that are used to produce microbially-safe drinking water. The information provided in this Guide represents current good practice and provides targets, both numerical and observational, which, if implemented, will achieve a reduction in microbial risk and help ensure the production of microbially-safe drinking water. Expert advice should be sought for WTPs whose design or operation varies substantially from those described in this Guide.