In the last decades, the frequency and intensity of cyanobacterial blooms have been of increasing concern. They have become a direct threat to the drinking water supply by clogging filters, bringing odour and unpleasant taste to the treated water and worst of all, causing elevated cyanotoxins, which can be difficult to remove, yet lead to severe health issues. This thesis aims to present a comprehensive knowledge base and tools for water managers and operators to understand cyanobacterial risk in their water so that bloom problems can be prevented or mitigated. Firstly, an adaptive approach for cyanobacteria management in drinking water supply is proposed, starting with an overview of this problem and resulting in a conceptual management tool design, a Cyanobacteria Management Tool (CMT) by which multi-indicators for actions are provided. Secondly, the magnitude of this problem in Swedish freshwaters was studied both on a national and local scale, including their geographical distribution, species dynamics, bloom seasonal pattern and their connection with eutrophication status, land use, and other factors. Thirdly, the studyhighlights impact of nutrients on cyanobacteria formation, including testing twohypothesizes, 1) if Total Phosphorus (TP) can be used to predict cyanobacteria risk and 2) if the Dissolved Inorganic Nitrogen and Phosphorus ratio (DIN/TP) is a better indicator for cyanobacteria risk than TN/TP. The results were also verified by a fullscale on-site experiment study of pre-treating eutrophic water at a local water treatment plant. Lastly, cyanotoxin detection challenges and strategies are presented. The key findings are:• Local target levels for TP for preventing cyanobacterial blooms are possible to be assessed by applying quantile regression analysis;• DIN/TP is a better indicator than TN/TP in predicting high levels of cyanobacteria; high levels of cyanobacteria coincide with DIN/TP