Cyanobacterial (blue-green algal) toxins are known to cause poisoning in humans, livestock and wild animals. Based on their toxic mechanisms, cyanobacterial toxins are generally categorized as neurotoxins, hepatotoxins or cytotoxins. The acute oral toxicities of these toxins vary substantially, with the saxitoxins being the most toxic having an LD50 of 60 μg/kg. By comparison, the acute oral LD50 for microcystin LR (the most toxic congener) and cylindrospermopsin are approximately 5,000 to 10,000 μg/kg and 6,000 μg/kg over 5 days, respectively. There are well known adverse health issues of cyanobacterial toxin poisonings. The most serious health consequences have occurred in Brazil with the reported deaths of people from gastrointestinal symptoms associated with exposure to microcystins and cylindrospermopsin. Increased number of symptoms has also been reported via exposure to cyanobacterial toxins through water-based recreational activities. Toxins may also be present in drinking water and thus guideline values are necessary to protect the health of the population. Guideline values are available for microcystins but not for saxitoxins, cylindrospermopsin or deoxycylindrospermopsin. Considerable research is being undertaken currently on more fully understanding the mechanisms of toxicity of cylindrospermopsin to enable relevant guidelines to be established.