Climate change will bring new challenges for the Swedish drinking water supply, which was clearly pointed out by a national assessment in 2007. Heavy precipitation and increased frequency of extreme weather, including droughts and floods, will increase the risk of both chemical and microbial contamination of drinking water. This will increase the need for adaptation through investments in the protection of water sources, drinking water plants and wastewater systems. Previous studies and our own analyses within Climatools, show that variations in rainfall affect raw water quality. Analyses made on data from the Göta River shows that there is a clear correlation between rainfall and levels of fecal-related bacteria such as E-coli in raw water. Also water turbidity increases simultaneously with the levels of indicator bacteria a few days after heavy rainfall, and can be used as an indicator of pollution. Protection of water sources and upgrading of sewage treatment are basic measures of adaptation. Due to the increased microbial risks in surface water, each waterworks must be equipped with enough safety barriers to prevent impact on the quality of the outgoing drinking water. It is appropriate to use not only disinfection with chlorine and ozone, but also ultra filtration and/or UV light. The impact of heavy rain on groundwater is most likely by a reduction in the aerated soil zone which is especially important to reduce the amount of infectious agents when water penetrates down to the water table. This report is a tool for those who work with climate adaptation at the local level and who want to include drinking water supply. The authors provide an explanation of what can happen with drinking water quality in a warmer and wetter climate and presents the various measures that can be deployed in Sweden. This report includes a checklist for the stepwise evaluation of the risks.