Cyanobacteria are the only group of phytoplankton to show a clear increase both in bio‐mass and relative contribution to total phytoplankton biomass as temperate lakes become eutrophic. Correlative studies indicate that this increase begins at total phytoplankton biomass levels of 3–5 mg fresh weight l‐1or spring total phosphorus (TP) concentrations of 25–30 μg l‐1. Above TN : TP ratios of 30, cyanobacteria tend to become rare, but below this value they may or may not dominate. Better predictions may be possible by attempting to remove the influence of the refractory dissolved organic nitrogen and phosphorus fractions by using the TN‐DON : TP‐DOP ratio. While experimental manipulation of N : P ratios in enclosures or entire lakes may often stimulate or suppress relative cyanobacterial biomass, laboratory studies do not clearly link low N : P ratios with cyanobacteria. Evidence from correlative studies, long‐term records of individual lakes, and experimental manipulations of nutrient loads suggest that other factors such as temperature, mixing regimes, transparency, and iron or carbon availability may influence cyanobacterial dominance in lakes.