This chapter provides a general introduction to key traits of cyanobacteria that support their proliferation in aquatic ecosystems. Under acidic conditions, cyanobacteria are rarely found in sufficiently high cell density to cause detectable levels of cyanotoxins. As in planktonic cyanobacteria, both toxic and nontoxic genotypes of a given species usually coexist within mats and cyanotoxin content can be highly variable spatially and temporally. Nitrogen fixation in most taxa is located in specialised cells, the heterocytes. Cyanobacteria produce more gas vesicles under low irradiance, thus increasing their buoyancy. For Microcystis colonies, models describe the vertical migration of cyanobacteria. Higher concentrations of cyanotoxins occur either in the topmost water layers, especially when buoyant taxa like Microcystis are dominant, or in a metalimnetic layer in the case of Planktothrix rubescens. Cyanobacteria therefore are efficient at light harvesting and generally have the capacity to grow at very low light intensities.