1. Metacommunity ecology addresses the situation where sets of local communities are connected by the dispersal of a number of potentially interacting species. Aquatic systems (e.g. lentic versus lotic versus marine) differ from each other in connectivity and environmental heterogeneity, suggesting that metacommunity organisation also differs between major aquatic systems. Here, we review findings from observational field studies on metacommunity organisation in aquatic systems. Species sorting (i.e. species are filtered'by environmental factors and occur only at environmentally suitable sites) prevails in aquatic systems, particularly in streams and lakes, but the degree to which dispersal limitation interacts with such environmental control varies among different systems and spatial scales. For example, mainstem rivers and marine coastal systems may be strongly affected by mass effects'(i.e. where high dispersal rates homogenise communities to some degree at neighbouring localities, irrespective of their abiotic and biotic environmental conditions), whereas isolated lakes and ponds may be structured by dispersal limitation (i.e. some species do not occur at otherwise-suitable localities simply because sites with potential colonists are too far away). Flow directionality in running waters also differs from water movements in other systems, and this difference may also have effects on the role of dispersal in different aquatic systems. Dispersal limitation typically increases with increasing spatial distance between sites, mass effects potentially increase in importance with decreasing distance between sites, and the dispersal ability of organisms may determine the spatial extents at which species sorting and dispersal processes are most important. A better understanding of the relative roles of species sorting, mass effects and dispersal limitation in affecting aquatic metacommunities requires the following: (i) characterising dispersal rates more directly or adopting better proxies than have been used previously; (ii) considering the nature of aquatic networks; (iii) combining correlative and experimental approaches; (iv) exploring temporal aspects of metacommunity organisation and (v) applying past approaches and statistical methods innovatively for increasing our understanding of metacommunity organisation.Academy of FinlandMaj and Tor Nessling FoundationBrazilian National Council of Technological and Scientific DevelopmentFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Finnish Environment Institute, Natural Environment Centre, Biodiversity, Oulu, FinlandDepartamento de Ecologia, Universidade Federal de Goías, Goiânia, GO, BrazilUniversity of Helsinki, Helsinki, FinlandTvarminne Zoological Station, University of Helsinki, Hanko, FinlandInternational Council for the Exploration of the Sea (ICES), Copenhagen, DenmarkDepartamento de Ecologia, Instituto de Biociências, UNESP –Universidade Estadual Paulista, Rio Claro, SP, Brazil, Department of Geosciences and GeographAcademy of Finland: 273557Academy of Finland: 273560FAPESP: 2013/50424-1