The zooplankton is extremely various and comprises adult and juvenile forms belonging to all phyla. Very important components of the zooplankton, from both phylogenetical and ecological point of view, are the unicellular organisms. Nevertheless, their systematic arrangement is particularly complicated and debated. In the past, the unicellular organisms devoid of chlorophyll were classified within the Animal Kingdom and called “Protozoa”; on the contrary, to date they are classified in the Kingdom “Protista” together with the unicellular organisms provided with chlorophyll. According to the recent taxonomic classification, in this paper are described the unicellular zooplanktonic organisms, also considering shortly some morphological and ecological aspects.