Cyanobacterial blooms have been reported in waterbodies all over the world. The occurrence of blooms could create water quality problems, as certain species of cyanobacteria are capable of producing toxins –microcystin, anatoxin, saxitoxin or cylindrospermopsin– that have been implicated in cases of human illness. During the last three years, 35 Spanish water reservoirs were sampled in order to assess the occurrence and concentration of cyanotoxins. Results show that in the 70% of the samples with cyanobacteria, the microcystin concentrations were higher than the World Health Organisation recommended limit for drinking water (1 μg/L). In the next four years CEH will continue these studies in Spanish drinking water reservoirs.