Cylindrospermopsin (CYN), a cyanotoxin produced by harmful algal blooms, has been reported worldwide; however, there remains limited understanding of its potential risks to surface water quality. In the present study, we reviewed available literature regarding the global occurrence, bioaccumulation, and toxicity of CYN in aquatic systems with a particular focus on fresh water. We subsequently developed environmental exposure distributions (EEDs) for CYN in surface waters and performed probabilistic environmental hazard assessments (PEHAs) using guideline values (GVs). PEHAs were performed by geographic region, type of aquatic system, and matrix. CYN was prevalent in North America, Europe, and Asia/Pacific, with lakes being the most common system. Many global whole water EEDs exceeded guideline values (GV) previously developed for drinking water (e.g., 0.5 μg L −1 ) and recreational water (e.g., 1 μg L −1 ). GV exceedances were higher in the Asia/Pacific region, and in rivers and reservoirs. Rivers in Asia/ Pacific region exceeded the lowest drinking water GV 73.2% of the time. However, the lack of standardized protocols used for analyses was alarming, which warrants improvement in future studies. In addition, bioaccumulation of CYN has been reported in mollusks, crustaceans, and fish, but such exposure information remains limited. Though several publications have reported aquatic toxicity of CYN, there is a lack of chronic aquatic toxicity data especially for higher trophic level organisms. Most aquatic toxicity studies have not employed standardized experimental designs, *