A number of countries have developed regulations or guidelines for cyanotoxins and cyanobacteria in drinking water, and in some cases in water used for recreational activity and agriculture. The main focus internationally has been upon microcystin toxins, produced predominantly by Microcystis aeruginosa. This is because microcystins are widely regarded as the most significant potential source of human injury from cyanobacteria on a world-wide scale. Many international guidelines have taken their lead from the World Health Organization's (WHO) provisional guideline of 1 microg L(-1) for microcystin-LR in drinking-water released in 1998 (WHO 2004). The WHO guideline value is stated as being 'provisional', because it covers only microcystin-LR, for reasons that the toxicology is limited and new data for toxicity of cyanobacterial toxins are being generated. The derivation of this guideline is based upon data that there is reported human injury related to consumption of drinking water containing cyanobacteria, or from limited work with experimental animals. It was also recognised that at present the human evidence for microcystin tumor promotion is inadequate and animal evidence is limited. As a result the guideline is based upon the model of deriving a Tolerable Daily intake (TDI) from an animal study No Observed Adverse Effects Level (NOAEL), with the application of appropriate safety or uncertainty factors. The resultant WHO guideline by definition is the concentration of a toxin that does not result in any significant risk to health of the consumer over a lifetime of consumption. Following the release of this WHO provisional guideline many countries have either adopted it directly (e.g., Czech Republic, France, Japan, Korea, New Zealand, Norway, Poland, Brazil and Spain), or have adopted the same animal studies, TDI and derivation convention to arrive at slight variants based upon local requirements (e.g., Australia, Canada). Brazil currently has the most comprehensive federal legislation which includes a mandatory standard of 1 microg L-(1) for microcystins, and also recommendations for saxitoxins (3 microg L(-1)) and for cylindrospermopsin (15 microg L(-1)). Although guidelines for cyanotoxins and cyanobacterial cell numbers for recreational waters are in place in a number of countries, it is consid ered that there is currently insufficient information to derive sound guidelines for the use of water contaminated by cyanobacteria or toxins for agricultural production, fisheries and ecosystem protection. In relation to the need for specific regulations for toxins for the US, the surveys that have been carried out to date would indicate that the priority compounds for regulation, based upon their incidence and distribution, are microcystins, cylindrospermopsin and Anatoxin-a. Additional research is required to support guideline development, including whole-of-life animal studies with each of the known cyanotoxins. In view of the animal studies that indicate that microcystins may act as tumor promoters, and also some evidence of genotoxicity and carcinogenicity for cylindrospermopsin, it may be appropriate to carry out whole-of-life animal studies with both toxicity and carcinogenicity as end-points. In relation to microcystins, it is known that there a large number of congeners, and the toxico-dynamics and kinetics of these variants are not well understood. Further research is needed to consider the approach to take in formulating health advisories or regulations for toxin mixtures, i.e. multiple microcystins, or mixtures of toxin types. An important requirement for regulation is the availability of robust monitoring and analytical protocols for toxins. Currently rapid and economical screening or quantitative analytical methods are not available to the water industry or natural resource managers, and this is a priority before the release of guidelines and regulations. There is insufficient information available in a range of the categories usually required to satisfy comprehensive risk assessment process for the major toxins to currently adopt any of the international guidelines as regulations in the US. The major limitations that need to be overcome include: the capacity to deal with multiple toxin congeners, the absence of robust analytical methods for compliance monitoring, and the absence of certified toxin standards to support analyses. However, the current WHO provisional guideline for microcystin-LR, or the other national guideline variants that are based upon it, (e.g., Canadian, Australian) may be appropriate to adopt as a health advisory in the short-term, while regulations are developed. The bathing and recreationa water guidelines developed in other countries could also be translated fo use as recreational water guidelines situation in the US.