Aquatic plants contribute to maintaining key functions and related biodiversity in freshwater ecosystems, and to provide the needs of human societies. The way the ecological niches of macrophytes are determined by abiotic filters and biotic ones is considered. A simple, broadly applicable model of the distribution of growth forms according to abiotic filters is proposed. The consequence for the dynamics of plant communities and the main threats to macrophyte occurrence and diversity are discussed. Key concepts: Macrophytes contribute to maintaining key functions and related biodiversity in freshwater ecosystems, and to provide the needs of human societies. Most aquatic species are phylogenetically descended from terrestrial plants that have later adapted to aquatic life. Water plants are usually poorly lignified, as water preserves plants from gravitational stress, and are characterized by the presence of aerenchyma, which increases oxygen flux from shoots to roots, and by a large leaf surface, together with a thin cuticle, which increase contact with water and carbon uptake. Macrophyte dispersal relies partly on water drift, and thus on seed buoyancy and on the ability of plants to break themselves up and regrow from broken dispersed fragments, and partly on endozoochory by animals (mainly birds). In freshwater ecosystems, the production, community composition and life-history traits of macrophytes are governed by the availability of carbon, nitrogen and phosphorous. Water movements (waves, flow velocity) tend to select streamlined or prostrate growth forms, and contribute to the dispersal of seeds and vegetative propagules. Moderate disturbances (by floods or drawdowns) decrease biotic interactions in aquatic plant communities and as a consequence favour biodiversity and decrease successional rate. Free-floating and tall species with floating leaves are the most competitive for light, and usually dominate macrophyte communities when nutrient levels in the water are sufficiently high. In the framework of global change, eutrophication of freshwaters, together with decreases in natural disturbance regimes in river floodplains, increases in groundwater abstraction and the increase of temperature may all contribute in decreasing macrophyte biodiversity by favouring competitive and invasive species, to the detriment of ruderal, stress-tolerant poorly competitive ones.