Zooplankton organisms comprise very important elements of the structure and function of marine and freshwater ecosystems, not only as consumers of primary production, but also as food items for juvenile stages of several fish species. Moreover, its sensitivity to both man-made and natural changes makes zooplankton quite suitable for assessing alterations in the trophic dynamics and the ecological state of aquatic ecosystems related to changes in nutrient loading and climate. Multi-scale, spatial and temporal relationships between zooplankton variability and environmental heterogeneity are still not satisfactorily understood due to the complexity of the different aquatic ecosystems (considering both biotic and abiotic elements). Thus, the ambition of the present edition is to contribute to the understanding of the role of zooplankton by investigating ecological aspects such as the species diversity, their spatial distribution and seasonal dynamics in relation to the environmental influence in various aquatic ecosystems around the world. Topics discussed in this book include the understanding of the role of zooplankton in the transfer of pollutants through trophic food webs; plankton models to explain red tides; spatial patterns of trophy and zooplankton communities in a tropical urban reservoir; the zooplankton variation in five Greek lakes; the zooplankton community in a nuclear power station cooling reservoir; the spatio-temporal dynamics of cladocera and copepoda in the Danube River; the gelatinous zooplankton in the Namibian upwelling region; and the zooplankton community in relation to the environmental factors in a solar saltern.