Zooplankton is a set of animals inhabiting the marine and fresh water column, which are not able to actively resist transport by currents. The most common representatives of zooplankton are lower crustaceans (daphnia, cyclops) and rotifers.

Zooplankton traditionally includes also rather large heterotrophic protists - unicellular and colonial. Representatives of most types of animal kingdom are found in the composition of zooplankton. In most water bodies the most numerous group of zooplankton are small crustaceans. Zooplankton also includes larvae of many animals, pelagic fish eggs. Zooplankton organisms feed on phytoplankton, bacterioplankton, detritus or smaller representatives of zooplankton. If organisms spend their entire life cycle in the form of plankton, they are referred to holoplankton; if animals spend only part of their life in the form of plankton (usually the larval stage), they are referred to meroplankton. Zooplankton is the basis of food chains in biocenoses of water bodies, especially marine ones. It is a link in food chains that connects phytoplankton, which form the primary production, with larger nektonic and benthic animals.

Some of the most common organisms of freshwater zooplankton, common in standing puddles and accessible for observation with a magnifying glass, are daphnia and bosminids, diaptoms and cyclops.